If you can't find a black r/c foam bumper, you might consider dying it black. There are products available that are made specifically to dye urethane foam. Also there are products designed for black automobile trim that might work as well.
I would suggest doing a search locally for companies that sell/produce Fiber Reinforced Plastic (FRP). They might be able to provide you what you need or at least direct where you can source what you need.
As far as Vintage Tamiya parts/info, have you tried using http://www.tamiyaclub.com
.? Since it is a Tamiya specific website, someone there might have what you need.
Hope these ideas help.