Also, to keep the betta warm on the journey ( if more than around half an hour ) a small heat pack will be needed . In which case the best way to transport your betta is much the same way as they are posted when you buy one online.
Get a cardboard box that will fit your bag/tub with a little space all round to spare, line the sides and bottom with bubble wrap, break out your heat pack and put it on the bottom. Place a sheet of bubble wrap on top of the pack ( so it doesn't get too hot for him ) and then put him in his bag/tub of tank water ,and put It in the box. It should fit snugly and not roll or slide about in there.
Also if indeed using a bag to transport the fish, remember to tape the corners of the bag up against the sides, so he can't wedge himself into a corner and get stuck ( I have seen fish do this before when panicking ), and leave a good sized pocket of airspace between the water surface and the top of the bag where you tie it .
Honeythore - This all sounds great, especially the cardboard box thing. I was worried about that. One quick question though - what's the difference between a cycled and.. not cycled filter? And what were you saying about disturbing a bacterial colony?
Mauricia - I think I'll be able to take as much of the tank water as I want. I'll be taking a bus home, about a 7 hour drive, so that's fine. I'll just keep it on my lap or someplace else close.
Thank you, EVERYONE, for the wonderful advice. I love this forum and will definitely be back with more questions when I start to build my community tank (here's to hoping) in April.