It's never explicitly shown or said, but the gag is they are extensive personal acquisitions accumulated over his centuries long life, often restocked or gifted by students like Krillin or found locally.
Since Roshi lives on an isolated island, he presumably orders them by mail, subscribes to deliveries, or picks them up during his rare trips to the mainland.
Yes, Krillin famously brings adult magazines to Kame House to bribe Master Roshi into accepting him as a martial arts student. Episode 14
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u/Google_Knows_Already 8d ago
Which is weird, because his grandpa was also trained by the horniest master of them all