PSCs!
 
Okay, so Personal Sketch Cards are small (2.5x3.5" standard trading card sized) and not from official sets... but they're cool, commissionable as whatever you want, and since I picked my own card stock the paper is absolutely the best for my markers and helps contribute to some of my best work. And Patreons at the upper levels can accumulate free PSC commissions as part of their rewards.