I read somewhere that you can 'fly' a sim (FMS was mentioned) with only a buddy box to ease into the sport on the cheap.

Is this true?

Is there a good source for buddy boxes?

Yes but you will need to make an interface cable instructions on the FMS site or buy one from one of the links in FMS. I have a 6 Chanel Conquest budy box no battery or Chrystal for $30.00 plus $5.00 shipping.
Here is another site with a good discription of the interface.

http://net-express.com/nexus/fmsinterface.html

A friend of mine is using a buddy box for FMS, he paid about $30 as well. He needs to stick battereis in it to have it work, I don't know if this is true of all of them.