Instagram is a nice place to start. Generally, plug makers follow each other and form networks so you can discover new ones through the ones you already follow! Some to consider:
Redfernadornments (Jaggersouth)
Etsy is also a great resource. Here are some shops I've had custom plugs done with great success:
ThirdEyeModified (I didn't pay for these, but won them in a contest. Quality was great nonetheless)
You can also have companies on BAF like Gorilla Glass and Diablo Organics custom make you jewelry.
The only thing you should keep in mind, is that "cheap" and "quality" are seldom side-by-side when it comes to custom plugs. You're going to have to shell out the money for QUALITY. That being said, many of the shops I linked have very fair pricing.