Hi Len!
I was at WDW last Nov. and spend a long time at Build A Pin. There were a large number of bases to choose from and they seemed to vary the selection from single, double to triple add-ons. For example, they had a pin base "My Favorite Character" that has a space for a single add-on. You can select from what looked like hundreds of character add-ons in various poses and costumes. I had no trouble finding my favorites (Tigger and Eeyore).
Nearly any customization is possible because any add-on will fit any base pin. You select your base and then check the number of holes it has, then select the add-ons you want. I did one base that was a beach scene and added on a Tigger and Eeyore to customize it.
When the CMs (who actually attach the add-ons for you) work on the pin, they will do a temporary set first to show you how it will look. Then you can change it if you want. Once settled, they permanently attach the add-on and you have a custom pin.
Depending on the size & pose of the add-on, you may not be able to do them in the order you want. I did one base of WDW with fireworks adding on 3 Mickeys (Steamboat Mickey, Fantasia Mickey and a standard Mickey). Because Fantasia Mickey was holding a wand in his hand, he sort of "forced" the position of the other two on the pin. No big deal -- it looks great, but it's something to consider when picking out add-ons.
I have not found anyplace on the web to purchase them. For me the fun was being there and running around the walls of base pins and add-ons picking and choosing (and spending a small fortune). Pin collecting is going to bankrupt me!

(But I'll have loads of fun on the trip to the poor house!)