Photon pair generator?

I don't know what that is but it sounds like an advanced research tool that would cost a lot of money

Depends on what you consider reasonable. If you're a researcher, Thorlabs has a couple for <$30k. You could also build your own, but you probably wouldn't be asking if you had the experience necessary to do this.

If you're a hobbyist, building your own would be an impressive project that would teach you a lot (look up spontaneous parametric down-conversion, a common way to create entangled pairs). It would also be pricey, as you would need an appropriate laser source (probably a nanosecond pulsed laser), a non-linear crystal like BBO, and a lot of miscellaneous optical components, etc. You can get this stuff second hand online for a lot cheaper than new, but it would still cost a lot for an individual. You would also need to characterize your output to ensure you're actually getting correlated pairs, which is outside of my expertise.