Yes you can buy white. But make sure you get the right stuff, there are 4 types.
White and clear for inkjet and white and clear for laserjet
Black and white or coloured is immaterial, its the inkjet paper that melts in a laserjet, that’s why you have to make sure you have the correct stuff. I get mine from Crafty Computer Paper in Hexham but there are other suppliers.
One more thing to note, with this method (but not using an ALPS printer) you have to seal the decals in before putting them in water else the ink runs off the paper! I use an artists matt varnish spray.