How do I find a steampunk paintbrush? I really want a steampunk Hissi but cannot find the brush

You can get them with Random Events or buy them from other users on the Trading Post or the Auction House. They sell for about 3 mil. If you're lucky you could get a Fountain Faerie Quest now that the faerie quest event is ongoing. Or if you're very lucky the Grey Faerie might ask for the Fountain Faerie to give you your prize.

 

You can also try your luck with nerkmids but I wouldn't advise it. The odds of getting a PB from the Vending Machine are low and getting the one you actually want even lower.