As far as I know the CLSC's don't rent out the pumps - the Kirkland Walmart is the only place I know nearby that does. Why are you pumping so much, BTW? Is it just to increase the supply? If so, more frequent nursing works better. If it's for the occasional outing, have you thought about buying a pump - might be better in the long run than pumping.
Milk supply can go up and down for various reasons - sleep, stress, fluids, baby sleeping longer at night or nursing more often...
When you say your breasts are sore, is it the breast or the nipple? Yes, pumping a lot can do that, but it can be due to other stuff too. Is it just one spot or all over? How much are you actually pumping during the day?