I know that every so often, a series with purportedly lost episodes will have some, if not all, of their missing episodes end up being recovered (with either the discovery of the first two seasons of the CBS-era Joker's Wild or the discovery of the 1968 NBC Primetime run of Hollywood Squares being the biggest finds, depending on who you ask).
Also, I know that in the two examples I just cited, both of those were found sitting in old archives (the Joker episodes being in the vaults at WCBS in New York, not sure about exactly where the Squares episodes turned up).
My question is, are there any other common sources for finds, aside from searching studio or network archives?