Upon removal of my AC vents today I noticed that the black foam stuff that keeps the vent's vertical position has worn away completely, where would I be able to get replacement foam? Can I just get that stuff at an autoparts store or do I need new vents?

Get some 3M 76 spray and get some insulting foam close to what use to be there from a fabric store or Home Depot even a packaging place. You should be able to repair it yourself.