Just a warning though:
THIS IS HIGHLY ADDICTING.
ONLY BEGIN DOWNLOADING FONTS IF YOU HAVE LOTS OF TIME TO TRY ALL OF
You thought I was kidding about the addicting part, didn't you? If you aren't seen by your family for a few days, or find yourself thinking about typography while you're supposed to be having a conversation with a friend or co-worker, an intervention might be worth a shot. You can even pick out the perfect font for the invitations and host it yourself!
If you're really brave (and have massive amounts of self control) here are a few of my favorite sites. All (or most) fonts are completely free, easy to download, and immediately available for use!