Quite a few vendors out there, quick summary of a few:
Abstract Ocean: Small items for interior, generally pretty cheap.
RPM Tesla: Similar to above, but also have some carbon fiber items. I have their carbon fiber dash replacement since i'm not a big fan of the wood that comes stock.
Unplugged Performance: These guys make some very impressive, but more expensive, parts that can really change the look of your car. We're talking full-carbon widebody kit for Model 3 if you want to go that far. They do interior/exterior and some performance items.
Revozport - Makes a full kit and some other items.
Vorsteiner - Has a kit with spoiler/diffuser/side skirts
Mountain Pass Performance: Make some really great race hardware for suspension/brakes and even a sweet electronic racing dash display. They don't make body kits or anything, but i felt they had to be included on a list of good Tesla vendors.
And then of course, but i hate to say it, you can always find things on ebay from China in all sorts of styles.