This sounds like a situation that should be set up either:
1. You buy exclusively through me. I add margin/labor, but you get qualified, tested materials.
2. I buy and test the first prototypes, and send them to you for verification. You get the fab data and testing documentation. This/these are the places I worked with, they're good [if applicable]. Whether you get a quality product, going forward, is up to you (I can help you with that, at standard rate of course).
3. Contract terminates with hand-over of fab data. You get it made wherever you like; quality is up to you.
It sounds like #1 is out because of tax, so I guess you'd want to pick 2 or 3. And it sounds like you want to be more responsible than 3, so it should be 2.
If you want your business to include fabrication and testing, then it sounds like you should pursue #1, and get the tax thing set up.
Tim