You can get some hair from stores etc but it will be a limited selection. Perhaps a specialty doll store may have a bigger selection. Other options are dollar store hair extensions, or harvesting hair from other dolls/ponies that have seen better days.
If you go with ordering online, here are several options that Skig has nicely compiled for her G3 to G1 Swap.
Dollyhair Nylon Also offers saran, polypropylene, and others.
CustomPony Nylon
My Little Customs Nylon - piper's site Offers saran and polypropylene, as well, in collaboration with Dollyhair.
RestoreDoll Saran Now offers 18-inch skeins of nylon, as well.