You used to could buy from sounddomain, etc., I don't believe it's made anymore. You'd have to search ebay. Like I said above, I personally wouldn't use it. It is NOT a musical sub, it's for test tones only. If you want to get any daily use out of it, you're better off looking elsewhere. I'd look elsewhere anyway, like two good 18s. It's also a 5,000 RMS sub, I doubt you'll get an alternator (or two, or four) that'll power the amp to run it well. It ain't cheap, either.
15s typically respond better up high. The benefit of 18s for SPL is sheer surface area, you have to have it for SPL. But, a large, heavy sub takes a lot of power to move and if the motor isn't strong enough, then the sub won't respond quickly enough to produce SPL. I don't recommend anything larger than an 18". It depends totally on your goals and personal taste on the 18s vs. 15s issue. An 18" isn't practical in an everyday (or any occasional listening to music) experience, but if your goals are SPL then they're likely right for you. Depends on the sub totally whether it'll sound decent w/ upper bass freqs.