How can I find a doc that allows birthing/laboring positions other than supine?

Most docs do. As long as u are not having problems most obs don't care what position you labor or birth as long as it works and does not harm u or the baby, please talk to your ob.
Do research. Interview several potential OB doctors in the beginning of the pregnancy to see which one has a similar philosophy as yourself. The one that you pick should allow you to labor as you like, as long as it is safe and the doctor knows when to intervene when it becomes unsafe.