How can I find a dentist that will accept my hmo as full payment?

Contract. Your hmo/dmo has a contract with various providers. These providers have agreed to treat you at a specific, reduced fee. That reduced fee includes copayments from you. This is the agreement your employer has with the insurance carrier and your dentist.
Not possible. An hmo gives a dentist such a little fee that it does not even cover the cost of the procedure, the materials, and sterilization of the instruments in most instances. The way a dentist is paid in a hmo network is by the number of people who sign up. The amount you are being charged is only available to you due to other people who signed up and did not come in for dental care.
You won't. Dentists that are part of an hmo are obligated by their contract to charge the stated co-pay if there is one. If they don't they are committing insurance fraud.
HMO. Ask your hmo for a list of preferred contracted providers.
Need to contact. Your insurer with a list of providers that accept your insurance. That would be the best resource for you.
Fraud. Hard to imagine, but that is insurance fraud. Your hmo has agreed to pay, let's say 90% of the fee. If they pay $90 and the dentist waives off the $10, really all you were charged was $90, and the insurance paid more than they should have.
Ask HMO for list. Hmo (dmo for dentistry) should have list of providers. Ask your insurance company for list. There may be a copay. Insurance was supposed to be hedge against catastrophic loss, not cradle-to-grave nanny state. If practitioner agrees to accept reduced fee plan they must reduce cost of doing business, which means compromise in level of care.