Mathmos lights are costly but you can buy them through Mathmos's web store. Lava Lamps, which Mathmos's founder invented, are another story. Since Mathmos sold Lava World the exclusive U.S. license to the technology way back, Lava World's more recently been keeping Mathmos lavas out of the U.S. market through what seems to be legal bullying.
Regardless, yellow and green Fluidiums (still my favorite kind of lava lamp, albeit quite large) are sometimes sold new in the U.S. (getting scarcer) for roughly $100 a pop. This is kind of a big deal since they're the only Mathmos lamps I know of which come wired for U.S. voltage. I think Plushpod's sold out right now, but Generate Design might have some. (I don't know of any other sources for new ones.) You may want to Google and contact both.
There's also Electric Planet. They sell the European lamps (so you'll need a step-up voltage converter) and are pretty much the one major webstore which ships to the U.S. Shipping can get really crazy and I've personally never bought from them before. But they sell practically every current lava EXCEPT the Fluidiums.
Otherwise try eBay (international searches) and step-up converters. Shipping can get insane on these, but it's among the easier ways to get Jets (impressive, small cylindrical lamps), Glitters (much larger flakes than most, really pretty) and most of the older / weird lamps. Mathmos lamps (lava and glitter) have nicer flow than just about anything, so it's nice to have a couple as extra shiny, centerpiece-ish lamps.
It's weird, but I just bought a Fluidium roughly a month ago from Plushpod (granted, they said they wouldn't be getting any more Fluidiums). So I wouldn't put it past some other U.S. or North Am stores still having some.
I am listing two brand new Mathmos Fluidiums on ebay, check it out if you are looking for these. They are really nice. I had a total of four. Of the other two: I already sold one and decided to keep one.