Yep, it's possible to own one. There are a few Hydro Thunders for sale in California right now, if you go use
this search engine at NamcoArcade.com. The going rate seems to be $1895, but I'm not sure if that's a single machine or a twin (IIRC, the two-player setups are two singles hooked together with an ethernet cable). I don't see any Sega Rallies there, but there might be some in neighboring states if you're willing to look that far. Sega Rally would likely be cheaper, because it's a few years older.
If and when you
do get a machine, you'll have to take care of a few things -- you might want to rig up a simple button to ring credits, or you could force your friends and family to dump quarters into it.
You'll also want to take better care of it than you would a simple console machine, because arcade cabinets aren't as fool-proof as console machines; they're more like big, wooden PCs. So, that means regular dusting and cleaning. You'll also eventually have to replace things like button switches, and light bulbs...farther down the road, bigger things like the potentiometers in the wheel/pedals/throttle-stick, or the monitor board, or the monitor itself may fail, and those will need to be replaced, but if you keep up maintenance and/or don't use the machine very often, they shouldn't be a problem.
By the way, in case you're wondering how I know all this stuff -- I work at an Arcade.
