I'm pretty sure I have Poland syndrome (one breast) and I would like to get diagnosed , how do I do that?

From your description, you likely have Poland's syndrome. Poland's can present with only breast development discrepancy or may involve underlying muscle and bone. The best way to get an opinion is to call and make an appointment with a board certified plastic surgeon. Look for one who does a lot of breast surgery.
It is not likely that you will have insurance coverage for breast size discrepancy but it depends on your company. Check with your insurance company and ask them for a written policy explaining your policy's coverage. I specify written policy because the person who answers the phone will not know the details of Poland's and may give a superficial answer. Get everything in writing.
Good luck to you.

Women with Poland syndrome may experience chest deformities such as missing or underdeveloped pectoral muscles and asymmetric or underdeveloped breasts. Some women may also have a missing nipple-areola complex. As such, this should be considered as reconstructive surgery. Attempts at insurance coverage can be done. Diagnosis is made by confirming absence of pectoralis major muscle.