Most enlargers use the same type of mount for their lenses. It's a 39mm thread mount, the same as used by old Leicas and a few Soviet SLRs. For this reason, lenses are mostly interchangeable between brands of enlargers. (There are a few enlargers with oddball mounts or other special needs, but they're rare.) Try measuring the lens mount; if it seems to be about 39mm, you'll probably be able to use most common lenses without too much hassle.
The general rule of thumb is to use a lens that's the same focal length as a "normal" lens for your film type -- so a 50mm lens for 35mm negatives, for instance. Many inexpensive enlargers are meant to handle up to 35mm negatives, and no bigger.
As Neal says, lenses are cheap on eBay these days. Most enlarger lenses have 4 or 6 elements, with 6-element designs being preferred. (Some really cheap ones have just 3 elements, and I know of one 5-element lens.) A $5 eBay bargain is likely to be a 3- or 4-element lens, but you can find used 6-element lenses for $50 or less if you don't buy the first one you spot. For just casual "playing around," you might as well go cheap; you can always upgrade to a better lens later. Even most cheap 4-element lenses can produce decent prints; the biggest advantage to the 6-element designs are in better performance at other than the optimum aperture.