there isnt anything like there is for gijoe and other military action figures.
i asked some star wars customizing groups i'm in for a dlt-20a ig-88 rifle because i dont want to lose my tvc ig88 rifle by giving it to a custom:
i've resolved to 3d print it.
these groups were unhelpful answering that and if there is someone selling cast parts, they're a well kept secret.
starwarsgeek was one for heads, but not weapons. he's been on a hiatus due to a myriad of reasons and is playing catch up (and trying to repair his reputation too because he was selling casts he didnt have to pay for healthcare costs and other various things that popped up and got swamped.)
the only other option i can think of, i wouldnt buy from them in a million years. i dont even think tehy're allowed here to sell anymore.