Junk shops and mom n' pop motorcycle shops usually have this kind of thing.
Off the top of my head I know of 2 in Valencia, somewhere around the Palawan Pawnshop and one on Smith Street past the market going down to Bacong.
I'm not sure about Dumaguete but around the main Russi dealership there seems to be a lot of store fronts that are full of garbage/junk. I'm not sure if that is what they are selling, if they are just lazy store owners or if it is just a nasty residential area but that's where I would start. Russi may very well have a used tire section too.