Other than season 11, there aren’t any season sets available. Mostly four-episode set with selections from multiple seasons.

Vol. 35 is pretty cheap right now: https://www.amazon.com/Mystery-Science-Theater-3000-XXXV/dp/B018WQBNY0/ref=sr_1_1?s=movies-tv&ie=UTF8&qid=1544074691&sr=1-1&keywords=MST3K&refinements=p_n_format_browse-bin%3A2650304011

So is Vol. 24, which is a really good set: https://www.amazon.com/Mystery-Science-Theater-3000-XXIV/dp/B007RMQ4F4/ref=pd_sbs_74_4?_encoding=UTF8&pd_rd_i=B007RMQ4F4&pd_rd_r=24fb2f79-f919-11e8-a598-6b142cabb10b&pd_rd_w=tFD1W&pd_rd_wg=vtU0D&pf_rd_p=7d5d9c3c-5e01-44ac-97fd-261afd40b865&pf_rd_r=F9D6EB5PWBA7N2VCWE3D&psc=1&refRID=F9D6EB5PWBA7N2VCWE3D

And the 20th anniversary set: https://www.amazon.com/Mystery-Science-Theater-3000-Anniversary/dp/B001IO07XM/ref=sr_1_10?s=movies-tv&ie=UTF8&qid=1544074795&sr=1-10&keywords=MST3K&refinements=p_n_format_browse-bin%3A2650304011

The singles collection is a little pricy, but it’s got a lot of great episodes: https://www.amazon.com/Mystery-Science-Theater-3000-Collection/dp/B079NCCK2M/ref=sr_1_3?s=movies-tv&ie=UTF8&qid=1544074856&sr=1-3&keywords=MST3K&refinements=p_n_format_browse-bin%3A2650304011