If you have very specific goals, you can jump on E-Bay and find the Colnago frame of your dreams.

However, if you are doing this on a budget, then a lot is just seeing what pops up. Craigslist/Thrift Stores/etc.

I'm seeing the limited selection of old Raleigh bottom brackets. However, if you are building a retro styled bike, you could use regular cotter cranks which would open up quite a few possibilities.