Depends what you want it made of.
There's a seller that regularly sells whole sets of Sintra armor on ebay, and also does the back plate on it's own (also in sintra). They go for, like, $20 - $30.
I've asked them a couple of times and they don't ship to the UK, which is a complete pain in the backside, as it's the bit that I need :facepalm
Failing that, there are several good prop makers who will make them from Vac formed plastic or fibreglass. Search and you shall find.
Hope this helps.