That is, is you're not looking for a really top-shelf super nice one that you're going to use every day as a chief moneymaker. For an occasional use one, yeah hit eBay.
There are literally dozens and dozens of sliders there (maybe even hundreds), that range from moderately priced, down to dirt cheap.
The good news is that quality is generally decent since none of these mom-n-pop sellers actually build the things totally from the ground up themselves... they are made of pretty readily-available off-the-shelf parts. Ergo, the quality tends to be pretty good.
There are also easily found industrial sources for the components (rails, carriages, etc.), so if you like to DIY you can put one together yourself with exactly the specs you want for a fraction of the price of a "store-bought" model. Rails and carriage systems are nothing remotely new, they have been around for ages... it's just fairly recently that the cinematography world has discovered them.
Fantastic Plastic Entertainment, Inc. fantasticplastic.com