Welcome!
ebay would be a common choice for post-1930 material. Start slow and build a relationship with a few sellers. Asking here is a good start, perhaps some folks will offer a few names of sellers they have had good experiences with (you might want to try asking over in the 'Modern US Stamps category).
Be careful if you move to the pre-1930 higher value stamps on
ebay, they are some shady sellers of that kind of material.
Don