The condensors are quite commonly found on ebay, but some of the condensors can be hard to find. Personally, I'd wait to get the proper thing from ebay than try to build one from scratch, since you don't know the proper curvature to the condensor.
Depending on what film format you want, you may be able to get the pieces together for relatively little money.
The negative carriers do show up on ebay also, and you may want to look at two othewr sources for parts:
Both have high quality products, but you'll pay a premium for them. At least they are an excellent source of info on these excellent enlargers.
I recently completed an 8x10 conversion for my 138, and think the chassis of these enlargers are about the best built and designed I have ever seen.