I don't have a 3D printer, so when I find something of interest to print (e.g. a case for some hackerware like PyGamer) I typically use Shapeways (https://www.shapeways.com/). I don't like it because I feel cost of materials and shipping, etc is pretty overpriced for even small pieces. Only reason to use it might be for large print pieces or for non-plastic materials.
Unfortunately said prints are in storage as I was in the middle of reorganizing and cleaning my place when COVID19 hit.
Edit: found a photo of a ring printed in sterling silver from Shapeways after about 5 months of wear, which reminds me of another con and another reason it's best to learn how to 3d model. If you try to get a print w/ someone else's files hosted on Shapeways, you can only use the materials the author has authorized. So for example, you can't print jewelry from someone else in say a tarnish-free material like platinum unless they say so.