While not always listed as CPO, you can still find used (formerly LEO Agency) SIGs on sites like CDNN, AIM Surplus, and, as mentioned, Recoil Gunworks. Probably elsewhere as well.

My first CPO was a 220st that looked new. It was in the early 2000’s but I think I only gave $525 for it. It’s one of my favorite guns. The CPO program is something that I really liked from Sig.