Just look on ebay or anywhere second hand.. I would go for an 40mhz if i where you.. But that depends on what you want. Do you want more a system that is more same speed as an 286? Or do you want a 386 system for just running games that runs fine on 386 and the beginning first version of the 486 ..
If you want the same speed as an 286 system, you have a choice to go for an 386 SX 25 /33 mhz system..need it more slower you also can take an SX 16 mhz processor..
For the last option i prefer to go for an DX 40mhz version..
40mhz precessor are today easier to find processors, sometime they are already soldered to the motherboards.. Only have to look if its an 40mhz version..
SX motherboards are easier to obtain.
In my opinion, an DX 33mhz is sometimes a little bit to slow, so that why i prefer the 40mhz one.
Iam only think, that you are a little bit to late with building this older computer.. Every year the prices will go higher on parts.. So i really dont know how much you want to spent..
But i can tell you, it can be very expensive if you arent beware of that.. my suggestion, try to find some stuff local.. it could be much cheaper then using ebay.
Finding an nice AT case it the most difficult part, most is already been processed by metal collectors.. So it would be very hard to find a nice looking, non / limited damaged case..
The motherboard could be expensive, but if you go for a cpu solderd onboard, you already got the processor and motherboard..
The other hard part is finding 30 pin simm memory.. There is still available yet, but very limited..
The rest of the parts is much easier to get.. So good luck on building that system!.
~ At least it can do black and white~