COMMUNITY

GOlfsmith, or even Callaway itself

I'm not sure Callaway will sell you heads by themselves. Golfworks.com sell a decent amount of components and golfsmith does as well, although not callaway heads. Both of those has a wide variety of shafts and grips to pick from.
You might be able to find a set of Callaway heads on ebay otherwise you might just have to buy the clubs you want disassemble them and reassemble them to whatever length, shaft and grip combo you want.
Another thing you might want to do is go down to golfsmith, and buy a single head, they often have some random ones on clearance for a couple bucks, whatever shaft fits the head either .355 tapered or .370 parallel, and a grip. You might want to build a practice one to see if it is something you want to get into before you spend a lot of time and money to track down Callaway heads.

OEM do not sell components.
Component companies sell parts, but not OEM components, they do sell clones or something similar. Shafts and gips could be had since the OEM bought their from the same manufacturers but the heads are pattern right could only go through the OEM like Callaway, Nike.....

For components, there are lots of good reputable shops, do a search and then look at their sites for information. The larger shops have more choices and more detailed description on the products.

If there is special needs you could email me for additional information.

Golfsmith has all the equipment, heads, grips, glue, grip tape,etc.... My dad and I have been making clubs ever since I can remember. I still use them today for all my component needs.