I've never found water pump brands to make as much difference as some other parts. With that in mind I'd hunt down a discount coupon code for Advance Auto parts and buy whatever they had, maybe needing to buy more stuff to reach a certain total at checkout to get the best discount %, but if you need a lot more large or heavy parts for something else, see what Rock Auto has to offer.
It's a pretty easy part to replace. I list a bunch of steps but most are self-evident while doing it... just what needs to happen next, sort of thing. Drain coolant either from the radiator drain valve or if it looks like forcing it open would damage it due to rust, or just to be safe, pull off the lower radiator hose instead. Have a pan or tub to catch the coolant. Consider whether it's new enough to reuse or if you want new coolant. Filter sediment out if you'll be reusing it. An old t-shirt over a bucket will work to filter it.
Get a wrench on the belt tensioner and give the belt slack, to slip the belt off the pump pulley. Look at what's in the way of pulling pump out an inch or so to clear the pump impeller from the engine, and unbolt anything in the way. Unbolt pump. You may not need an inch, it varies per pump housing design.
Take the pulley off and transfer onto the new one. Clean any rust off the pulley surface where it would contact the belt. Clean off the mounting surface with a plastic putty knife & wipe away any remaining residue. Apply your choice of sealant then install it. There's probably a gasket in there too. Preferences vary, I just use some regular automotive RTV sealant.
Let the sealant sit to cure for a few hours. Put radiator hose back on if you hadn't already. Fill radiator with coolant. Let engine run till thermostat opens, bleed coolant system to get bubbles out and top off coolant, see the following topic:
http://www.explorerforum.com/forums/showthread.php?t=251416
Also do anything I forgot to mention, like taking hose(s) off the pump housing before removal and putting them back on after installation. Check it a few days later, after a long enough drive to get the engine good and hot, to see if the coolant level needs topped off again. Some people might say you should disconnect the battery before starting, but if the radiator fan isn't electric I usually don't bother for a water pump. Your call, it's not hard to disconnect and reconnect.
