I use Bratz, Barbie and Disney Princess Styling heads. They're just heads that have hair that girls are supposed to style (sometimes they come with makeup to put on the heads as well). The styling heads are designed to be brushed and played with over and over again so the hair is sturdier, while still being soft enough to look pretty when it's been styled and I've found it has a texture very similar to brushable pony hair. The actual Barbie/Bratz/Princess dolls have hair that tends to be thinner and breaks MUCH easier when you brush it or try to pull the plug through the hole (Barbie doll hair is especially bad).
Plus, when you use the styling heads to make hair plugs you end up with enough hair to do somewhere between 10 and 15 ponies each. I buy mine at a local thrift store for $1-$2, so it's definitely worth the time it takes to clean and make the plugs. And it's nice and long so it can be usually be used for styling size ponies as well as regular brushable sized ones.
The colors of the styling heads tend to be browns, blondes and reds, so if I want purple, pink or blue, I use the larger Strawberry Shortcake dolls, the ones that are about the size of the Disney Toddler Princess dolls. Speaking of which, the toddler princess dolls also have hair that works well.
If in doubt, you can always buy one of the dolls at the flea market and try it out. If it doesn't work for you, you'll know for the future that it isn't what you want. If it does work, you can go get more.