One wonders what sort of budget the OP is working within?
I bought a "lightly used" Sigma 150-500mm, to compliment my recently acquired K-70, from a genuine retailer in the UK at the beginning of October for £350, so bargains are available.
Alternatively, the Samyang manual focus mirror lenses are quite economic, albeit the 800mm can be a bit of a fiddle to focus and doesn't come with a tripod mount or hood. (Yes, I've got one of those as well ... eBay s/h £100)
My other "long lens" recommendation/suggestion would be the Tamron Adaptall-2 500mm f/8 mirror. Well built, with tripod mount and hood and performs adequately when fitted with the matching Tamron Flat-field 2x converter #01F ... had one for years, now partially retired with the arrival of the Sigma
Good luck