I see 870 receivers in the EE whenever I look it seems. You're way more likely to find older Wingmaster ones, which will accept 2-3/4" shells. Some Wingmaster Magum receivers pop-up from time-to-time, but expect to pay way more. You're not as likely to find an Express receiver because for the most part...fewer people want the duller-finished barrels, or cheaper-wood stock sets they come with..so fewer guys parting those out.
I bought an 870P a few years ago from a CGN sponsor. Overpaid for it too. I like it..but if I was doing it again, I'd start with a regular old 870 Wingmaster receiver..and build one. I'd also likely get the whole works parkerized if the bluing was in rough shape. A minty receiver? Would be hard to pass on a classic "police" looking build.