Does anyone know where I can get cassettes for cheep?

I recently received a boombox from my dad as a gift, since I enjoy playing CDs and records. And most stores only carry cds or record and I can barely find any online. The boom box cam with a single tape and that was The Eagles. I'm mostly listening to the radio but its a mix of the state im in and the state next to it bc im almost on the border of it. I want mostly metal and rock albums. While I'm writing this im listening to Ozzy Osbourne nc he's on the radio. No More Tears 4 evr. But please help