Cat carriers work well to fit 2 average sized bunnies in. The slightly larger pet carriers are the best really, to give the bunnies a bit more room. I highly recommend the catit cabrio:
http://www.amazon.co.uk/dp/B002RKWS5Q/ref=twister_B00EUDYRU2 - it's really well constructed, nice and roomy, nice and secure and the top opening is really handy. You can also remove the door so you can leave the carrier in the hutch/run permanently, which means they don't learn to associate it with the vets and also provides a nice little hidey place. Obviously a bit too late to buy it off amazon though!
The best thing to do would be to get a cat carrier before you go, as you will need it for vets trips anyway. Avoid fabric ones as they can be nibbled through in seconds and don't provide any security. Top opening ones are very very handy, so I'd recommend one of those if you can. If not just a standard cat carrier will be fine, but make it as large as possible.
Depending on where you're picking them up from you may be able to borrow a carrier, or even buy one off them. Rescues often have an excess of carriers as obviously many people giving up their bunnies give all of their stuff along with it. I'd definitely give them a ring first before you go out and buy, just in case.