I have always found The Paintballstore to be very good and not only do they sell a variety of pump action markers but they also run pump action days at Holmbush Paintball so if you are local you have an ideal location to use your pump action marker should you choose to buy one.
if your not fussy about new then check the forsale section on forums and if nessisary post up in the wanted. the other option you have is get yourself a cheap autococker and a pump kit and hey presto your sorted if youve never played pump before i would reccomend starting off with open class and going for a autococker style marker. it will help you switch easier