Come on! All this talk about de-french threading stuff.....This stuff is not rare. It turns up on da 'Bay almost daily. Several companies have NOS or are still making stuff. It's rarely overly expensive unless you're after high end stuff. My one word of advise is don't buy incomplete bottom brackets unless you have access to a ton of spindles....
Been there....Done that!
For the rear derailleur as mentioned before......You can buy NOS Simplex Prestige for around $30. The SLJ 5500's will cost $50-$60 used which is compareable to NR (except the 78 date code for some reason). Even VeloOrange has a cheapo Simplex that's all metal for like $10! Headsets are usually going for around $30-$50 even for the nice ones.
A little tip...When searching ebay use the measurements for components like "35 x 1" for bottom brackets. Search title and description world wide. Sometimes people don't know what they have. Memorize what a SLJ looks like and browse for Simplex, Spidel and Gipiemmie derailleurs.
Hunt on French eBay too. I like to just type in velo ancien..... Sometimes you'll get lucky. Just watch the exchange rates so you don't get ripped.
I like google alerts too. I've found some neat stuff that way. Maybe use one of those Criagslist searchers that search Craigs worldwide......And ask if they will ship. I got some Topline cranks for $25 that way.
Geez...Probably shouldn't have given away my tips. I've noticed in the last year people are catching on to the SLJ one. The price has doubled or more in some cases!

Oh....And don't forget the Phil rings and Shimano UN72 trick! That can save some serious coinage on a nice sealed cartridge BB with an adjustable chainline.