I couldn't find a specific p/n for the proportioning valve, it's made up of a whole bunch of parts inside the combination valve. Someone who really knows brakes here will have to help you with that.
I got lucky and the 99 frame and tub I got to swap my 97 came with a spare.
I'd suggest a junkyard or Davey's Jeep or beg @mrblaine for something out of his special stash of parts... actual I think the last time he talked about this I think he mentioned it was nothing more than a T with a switch. To paraphrase inelegantly.