I am using nail polish remover which is essentially acotone. It is cheap, but you should be careful buying one that has no added ingredients (fragrances etc.) just acetone and water.
But as someone suggested before, you might be able to remove it without any solvant.
Also, I am fine using paper tissues, I haven't had any problems with them. I usually clean the CPU with a dry microfibre cloth after there is no thermal paste left.