In a big city most autoparts stores have it. Smaller places you may have to mail order.
Unlike regular bondo, I found it was better to use it sparingly and do several applications. It is tough to sand.
Two types:
Twice as strong as regular body filler. Half-inch, interlocking fiberglass strands create strength and make Bondo® Glass the ideal filler for repairing small (dime size or smaller) holes or rust-outs. The fiberglass strands in the filler allow you to cover holes and eliminate the need for backing strips, fiberglass mat, or cloth. Make repairing small holes easy with Bondo® Glass.
It's all in the name. Formulated with up to inch-long strands of chopped fiberglass, Bondo® Hair repairs 1" rust holes, shattered fiberglass, and dents where extra strength is required. At twice the strength of regular body filler, this filler is great for rebuilding missing pieces or vehicle restoration work. For your toughest repairs choose Bondo® Hair.