I would like to send a presentation letter to introduce myself and my wedding video business to the local wedding planners and venues, but I'm having a bit of writer's block. Does anyone know of a website with some templates that I can start from and get a better idea of what to write?

There is no template for this kind of thing.

Let me give you $0.02 of advice for free:

Unplug your router.
Unplug your phone.
Unplug any external hard drives connected to your computer.
Buy one of those $3 kitchen timers - the kind that you crank, and they go "tick-tick-tick" until a bell dings. Do not buy a digital timer. Do not use your iPhone. Do not ask your wife to tell you when 30 minutes have gone by.
Lock the door to your office.
Open a word processer.
Set the timer for 30 minutes.
Write the letter.
Do not open your resume for ideas of what to write about. Do not review your website for ideas of what to write about. Do not clean off your desktop. Do not call your grandmother. Do not edit your work. Stop screwing around and get back to typing.
When the timer goes off, save it and do not open it again.

The next day, go through the same rigamarole, but this time, edit what you've written. Do not delete what you've written and start over. Just edit.

In 3-4 cycles of this, you will have a letter that outshines any that you would possibly find in a template.

Excellent advice, reminds me of similar advice from my college English prof: "To write a good paper, write ten pages, let it sit a few days, and then edit it to five pages. Let it sit a few more days, then cut it down to two pages."