I use a rarity for custom 'mechs that ranges from "Ubiquitous" to "Singular." Availability falls off according to various factors, such as Clan/InnerSphere, neighbor, non-neighbor, no longer in production, etc.
As a general rule of thumb, any one 'mech that's not singular could wind up just about anywhere. Favors get traded, 'mechs get sold into and out of merc units, and sometimes a lot of scrap has a salvageable chassis in it.