Rob - I've actually seen CD-R copies of non-retail mixtapes on sale in gas stations and headshops (and those weird half-gas station, half-headshops that pop up in the city... you know, those places that sell pipes, weaves, east a hats, XXXL t-shirts and doo rags as well as watered down gasoline and off-brand cleaning supplies?)
It happens. But the dumb thing about that is that ANYONE with an Internet connection and a trial version of WinRar can make their own CD-R burns lf mixtapes.
Don't go through YouTube. Go through DatPiff or HipHopDX or one of the countless other sites that allows you to freely download the entire track list of a mix tape. Then burn that bitch to a CD.
There are also "retail" mixtapes, like the Drake/Future abomination, or Indigoism, or [insert more titles from the literally never ending list of mixtapes that just won't stop coming out here]. Those are available on iTunes and shit... sometimes google play.
Then there are the mixtapes that are really just albums, like If You're Reading This it's Too Late, which is a fucking *studio album that can be purchased at any Best Buy or Target* .... it gets pretty fucking ridiculous honestly.