I haven't tried to build one yet, but I can give you a cheap place to buy them at:
http://www.hobbyterra.com/ I have ordered from this company a couple times. Even though they are in Russia, my orders have always arrived fast. The only thing I can recommend is when you order try to pick out models that are easy to distinguish between.

When they ship the models, to save space the parts are all sealed in their plastic, and the boxes for the models are shipped flat. You need to glue the box together yourself if you want to shelve it.
I will not think twice before ordering from them again or recommending them to anyone.
Don