It's all the rage now to build your whole R2 out of CNC machined aluminum parts. A lot of builders have figured out how to either manufacture the parts themselves or they contract with a shop to do a run. Then every 6 to 18 months they do a run of 20 or so parts for all of the interested builders. So the custom is for you to watch the forums on Astromech.net for a long time and every once and a while a run comes up. Pay your money, wait another 3-6 months, get part in mail, then wait on other parts. Once you've got them all collected, put them all together with some fabricating, adjusting, and building. The kicker is that these parts are really expensive. Aluminum frames are going for $750, skins for a couple hundred, domes for $500, feet for several hundred, foot motors for $700, legs for hundreds, and so on. It looks like it is possible to assembly a fully machined aluminum R2 now by this method that's even better in lots of ways than the models they used on the sets. But it'll easily run you $5,000.
If you can do that, then that's cool. But here's my deal: I am a compulsive DIY guy. On my house, my dryer, my car, my patio, my kitchen sink, I figure I can pay a profession a few hundred bucks to do it, or I can spend the same cash on some parts, some tools, some research, and some experimentation and I'll learn how to do a decent job of it myself, and I'll have the skills and knowledge from now on. So what I am saying is, CNC machined aluminum parts are for pussies. Put your back into it and take the opportunity to make something better of yourself.
I have to say that with all due respect though because there are some brilliant machines out there, assembled by smart, clever guys and I have learned a lot from them.
So with that said, I'm going to go with a wood frame, styrene hand cut skins, hand fabricated details, hand built legs, etc. I'll have to buy some machined parts, I figure, like the dome and some details, but I'm going to try to DIY as much as I can.
Dan Baker, at: http://bakercraft.net/dan/r2d2/r2d2_index.htm
has got the right idea. I've learned a bunch from his site, and Victor Franco's too.