Research, research, research. Look through people's logs and other Supplement posts. There are loads of products that will claim to give a pump but from the logs you could try AP, P-Slin, plasmajet, Prime, etc., etc., etc.
I've used Xtreme and it did well in building muscle but I didn't get anywhere near the hardness/fullness that I did when using PowerFull so wouldn't be a choice if I was purely looking for muscle hardness.