Mathmos are more expensive, yes. I don't think they ship outside of the UK. I think your best bet will be private seller here, or international seller on eBay. Again, shipping will be expensive, but if you've got the $$$, I'm sure someone will ship you a lamp.
Older lava lite brand lava lamps are as good as current mathmos lamps and they are a lot cheaper since you are buying them as used most of the time. You can buy a lava lite brand lamp in the usa for about $35 usd off of ebay if you find a good deal. But to buy a mathmos it costs us americans well over $100 to buy it and ship it over here.
I know, I was just explaining to you the price differences we americans experience when buying a lava lite brand lava lamp versus a mathmos lava lamp.
Most knock off lava lamps come from china so if your into those you could buy those otherwise you will be paying double what the lamp is worth because of shipping. Basically importing anything from other countries is very expensive and that is probably your only option.