Teespring still works really well if you have quality graphics and original designs. You can always JV with a graphic design student (I pay my next door neighbor $1 per shirt) or hire a VA to do your designs for you. There are other alternatives such as Represent that work well too. It really is all about getting a targeted audience, building trust, and testing different designs. Anything humorous will get plenty of shares, likes, and comments. Find the RIGHT design and you can bank.
Careful with Facebook ads though...it can get pretty expensive if you don't have much experience dealing with it or don't have the time to test when to post ads, etc. You can also try to be niche driven and post directly to some FB groups but this takes some testing as you don't want to appear too spammy. Just with any other IM venture, make sure to engage with your audience before cramming sales down their throats.
