I used Microscale brand, they are found mostly in model train shops, I think. (I'm not at all sure). The directions say nothing about any special sprays or sealants, except the ones I talked about before. My dad works on model trains a lot and uses the paper, and he's never used anything but the solvents I talked about before (cheap, small bottles), and he's had models around forever and they've never had their decals run.
Also, with the sealant (cheap kind) over the top of the decal, it looks identical to a very crisp, very clear paint job. The sealant does make a difference for that.
It could be that the brand you're using is also used for some other really specialized application, and that's why they make it so that it needs the special sealant.