After my Sennheiser lav mic (the one that came with the G3) gave out, i went to buy a replacement:

Amazon.com: Sennheiser ME 2 omni-directional lavalier EW microphone: Musical Instruments (http://smile.amazon.com/Sennheiser-ME-omni-directional-lavalier-microphone/dp/B004UNQ4US/)

$140. Ouch!

I bought a couple of third party replacements, and really fell in love with the MIC-J 044 for $25:

http://www.amazon.com/Lavalier-Microphone-JK-Sennheiser-Transmitter/dp/B009YCI62Y/

Been using these for a couple of years and they've been good to me. Perhaps not as strongly built as the Sennheiser, but a really good mic for $25.

They go on and off sale at Amazon - I assume they come from China in a crate of 1,000 and whoever sells them does so until they sell out, then order another crate. They are currently off-sale and i find myself in need of a new one.

Anybody have a suggestion as to where to get one, or a replacement model? Makes me nervous to be in the field without a backup mic.

This is a possibility: Omnidirectional Mono Microphone (http://www.giantsquidaudiolab.com/gs/gs-mono.html)

They have a Sennheiser-specific model for $40. Disclaimer: I have no hands-on with this mike. There are some reviews on YouTube that you might have a look at....

Sorry, double post somehow. See below.

This is a possibility: Omnidirectional Mono Microphone (http://www.giantsquidaudiolab.com/gs/gs-mono.html)

They have a Sennheiser-specific model for $40. Disclaimer: I have no hands-on with this mike. There are some reviews on YouTube that you might have a look at....

Newegg.Com has this: Pro Lavalier Noise-Cancelling Condenser Lapel Microphone for Sennheiser Wireless Transmitter JK MIC-J 016 - Newegg.com (http://www.newegg.com/Product/Product.aspx?Item=9SIA3A310W0240) as well as a ton of other cheap lavaliers. {edit: on Amazon also}

Audio Technica makes one I use on my DSLR, and it is satisfactory but does not have the locking Sennheiser connector:

http://www.amazon.com/Audio-Technica-ATR-3350-Omnidirectional-Condenser-Microphone/dp/B002HJ9PTO/ref=sr_1_1?ie=UTF8&qid=1426970569&sr=8-1&keywords=audio-technica+atr+3350

The G2/3 bungled ME2 mic is a POS anyway.
Many folks use the Oscar SoundTech side address 801/802 (Tram like) and/or the end address (oil can shaped) TL40. Both are around $120 cheap (w/ 3.5mm locking plug) and highly regarded by most film sound pros..

You might look at Sound Professionals. For example, here is an AT831 mic with a 3.5mm TRS plug. That's a fairly well-known mic. Note that this is a cardioid. Price is $79.00 (very good IMHO), and you can get a better deal if you "join their free discount club."
Sound Professionals AT831 (http://www.soundprofessionals.com/cgi-bin/gold/item/AT831-SP)

They are also selling the AT3350 mic, without the normal bundled power supply (so it does need "plug-in power"). This one is omni, and price is $49.95 (so it would be cheaper to buy the normal powered version, cut off the power supply, and solder your own connector.
Sound Professionals ATR3350 (http://www.soundprofessionals.com/cgi-bin/gold/item/ATR3350-NP)

They re-package a few other AT mics, as well. And they also sell their "own brand" mics, although some of those appear to be simply a Panasonic capsule inside some heat shrink. IMHO I'd stick with name-brand mics with some known history.

I hasten to point out that the AT831 is a cardioid, and you asked about an omni. But since some other posters gave links to cardioids, I thought I'd toss this out for consideration. And if you shop around the Sound Professionals website, you might find an AT omni that would work for you.

The G2/3 bungled ME2 mic is a POS anyway.
Many folks use the Oscar SoundTech side address 801/802 (Tram like) and/or the end address (oil can shaped) TL40. Both are around $120 cheap (w/ 3.5mm locking plug) and highly regarded by most film sound pros..

Yes, the Oscar SoundTech mics are excellent values. I replaced the ME2 that came with one Sennheiser G3 with an OST 802 and liked it so much I replaced the other. Now I mostly use them wired, without the G3s. They sound just great -- just like the local TV news crews who are using Tram TR50s.

Somebody said something about cardioid lavs. Don't go there. Omnis are way less trouble and sound better. The only time to use cards is if the talent you pin them on is highly trained, competent, is dedicated to producing great sound, and in a truly noisy environment (factory, middle of a football playoff crowd in a close game, etc.). If you don't meet all four of these requirements stick with omnis. Just sayin'.

As Bruce stated, I concur.. cardioid lavs should be avoided except.... 'maybe' in conjunction with a sound reinforcement system (PA) where feedback is an issue.
the OST mics also have low cable/rubbing noise... advantageous to inexperienced users. The T40 however should be avoided for high SPL environments .

Are there other dealers for the Oscar SoundTech lavs besides what comes up in a search from Professional Sound Services? Not complaining, just curious who people are ordering from.

I bought my OST mics directly from Oscar. Excellent bang for the buck, IME.

You can get them direct from the builder in Edison NJ.
http://www.oscarsoundtech.com/
Customer service (Dave), is OUTSTANDING !!!
PS, I have no commercial ties to this company