i am all excited now! I may make a g1 big brother into a Big Mac if I can get ahold of some dirty blonde hair before signups are over XD
Do you have an actual baity big brother? They are kind of hard to find in bad condition and all of the big brother's are worth a bit of money. I would suggest to anyone making a custom pony (G1, 2, or 3) to do some research before hand. You don't want to destroy a valuable toy. Or just post here and someone will help you out.
As for hair for customs you'll want to check out www.dollyhair.com/mlpnylonhair.htm
This is the same hair Hasbro uses. It might take a week or so to get your hair so order as soon as possible once you know what you need. For a FIM pony one hank is more than enough. For a G1 one hank might be enough if you don't make the hair very long. You can get hair other places (dollar tree kids wigs for example) but if you want to color match exactly then Dollyhair is the way to go.