Any rear bumper from any of those 1/24 or 1/25 kits will do (just not a modern Beetle). It's going on a wall in a diorama so I'm not picky. Just don't want to buy an entire kit if somebody has one in their parts box.
This project is like peeling back an onion with the amount of details- every time I get one layer done, I noticed another layer of things in the same pictures I've been staring at for 5 years. Its weird what you DON'T see when you are looking for something specific. I just noticed the other day that the back door has a door closer thingy at the top. After doing a quick google search for some reference, I find it is in fact called a 'door closer', and the one in question is a 'vintage' style. Its one of those every day objects we've all seen a thousand times in our life, but never focused on. Fortunately for me, a 'vintage door closer' is shaped awfully similar to a piece of plastic model kit sprue in 1/25 scale!
Then there are the fan belts: turns out that 1/25 fan belts are pretty much identical to small hair 'elastics' (discovered thanks to my two daughters!). The bad news: they only had rainbow colored ones. The good news: found a package of 500 in black at the dollar store for... one dollar. Felt a little weird buying them as they are apparently made for African-American folks' hair, so I got a coke and some peanuts to draw attention away from it.