Part numbers are in the FAQs and Help sticky, most are in the Peep and Firesights link, the WGRS is listed on the retail parts price list page. The FP and FP-TK used to be in the H&R store, but it's gone, you just have to call and ask for it, the FP-TK was $60, dunno how much Remington wants for it now, but it was always less there, the $36 WGRS was $18 thru Gardner, the FP-TK is $82 at Midway now.
Tim