So...DISCLAIMER...I'm NOT making a living from 3D printing. I work at a company that does a ton of 3D printing.
My company does a lot of rapid prototyping of parts. And engineer gets and idea for a different design, they'll draw it and 3d print it to test.
Suggestion: Find a small manufacturing company in you area that does not have 3d printers and offer to be their go-to. All they would need to do is send you the print file and let you know what type of material. Charge a retainer and by the piece and offer quick turn around.
I will say, good 3d printers are pricey. We have 3 that ranged from 1k to 5k and our most widely used one (which uses dry powder and a laser) was 40k.
Second Suggestion: Find a niche and print on demand from an Etsy store. For example....there are stores that will print you a model of the U.S.S. Enterprise from Star Trek. I've seen stores that specialize in cosplay stuff...Bobba Fett's helmet, armor...stuff like that. The good thing about that is you don't incur the expense until after the customer has paid.
This is probably the quickest way to set up shop and start making money.
Regardless, you'll need to really learn the in's and out's. There are some really impressive printers out there but each has a decent learning curve.
Good luck!