I'm doing custom Christmas cards for a few clients. I simply posted on my town's various local Facebook buy/sell/trade groups. I gave some basic pricing points and included a picture of a completed card. I also made sure to mention that I only had 3 commission spots available and that it would be first come first serve; this was my attempt to generate a bit of urgency. I tried to throw out some key words, like "fun", "personalized", "unique", etc.
The biggest issue I've had with people is price. They don't want to pay more than $50 or so for this type of thing. Good luck to you!