Have you contacted Gray Guns? They've had various parts made in-house, as well as a very good source of SIG parts. Although they tend to have people send in their own part (customer provided) when I still owned SIG pistols.
Through some searching, I see they have/ had their own intermediate trigger.
http://grayguns.com/products/p-serie...diate-trigger/
I do know through the production history the trigger dimension changed, so their part may be what you can use.
I didn't check the links provided, but I did find a trigger y are ago on eBay, which was a custom tool d and also pricey trigger.