I guess these are mine in no particular order of importance. Hope they are useful.
1.) When people post a custom on the forum they often list the various parts they used and who they got it from. If you like what you see, it will point you in the right direction for parts. Contact the seller and find out their turn around time.
2.) There are many styles of heads out there for a particular figure, if one person doesn't have it, another one might, so always shop around and see who has what you want. You may be inspired by something you hadn't thought of before.
3.) I usually make a list of the parts I need for 2 or 3 figures I want to work on. Then I find out who sells what and how I can best order those parts to take advantage of shipping costs. It's cheaper to order a few heads or accessories all at once from a store.
4.)Ask questions and read the fine print. Are you buying a pre-ordered item that isn't in stock yet?? They are often delayed for reasons the seller can't control, so don't be PO'd when it doesn't arrive right away.
"Steel-like jaws clacked away, each bite slashing flesh from my body - I used my knife and my hands, and when they were gone, my bloody stumps - and yet the turtles came."