Reviving this thread because a lot of people are asking questions now about costuming:
Livejournal BSG Costuming was mentioned earlier. I have never really used it but it looks decent.
The Colonial Fleet I would call this the most comprehensive source for costuming. You might have to register to get access to it, but if you are serious about costuming it is worth the effort.
Galactica Hero This is a very good resource as well. Lots of cross information from the last place. The guy that runs this site has a lot of the patches made up and makes many of the costumes you will need. I see him at every con around Dallas and is the source for pretty much all of my parts for costumes that I cannot get off the shelf.
Tank tops This was provided by DawnAZ in another thread. Go for the brownish ones as they are most screen accurate if you are into that sort of thing.
If you have any questions about costuming you should be able to answer ANY of them at one of those three links. If you still have questions, feel free to PM me and I will try to help you.
I handmade a deckhand costume and put together a marine costume so I have a little experience in BSG costuming.
Hope this helps folks out especially those looking for parts for the national meetup.