Where can i find a replacement for this?

I don't think you will find a replacement online. I think that piece (the cover for center rear seat belt in the package tray) is part of the seat belt assembly itself. While it snaps into the rear package tray, there is no way to separate it from the seat belt (unless there was a slot in it to allow the seat belt to be slid into the cover). There are a couple 1st Gen Avalon's in a salvage yard by me - I don't remember what color interior they have. If the belt can be removed from the opening in that cover, I could try to remove it from the package tray (but they tend to get brittle with age, especially something that has glass above it to let the sun get at it). If is is part of the entire seat belt assembly, then the only way to replace it is to replace the entire seat belt assembly.

Could others who have a 1st Gen Avalon advise if that package shelf seat belt cover can be separated from the seat belt?