It’s one of many things you won’t find at the big box stores. I don’t know of anywhere in the West Metro that would have one, unfortunately.
Basically, a stringer is just a long cord with pockets at the ends for the tips of your bow. Put the string on your bow, with one loop in the nock groove of the bottom limb and the other loop slid up the top limb. You put a pocket over each tip with the bow’s back pointed up (the back is the side that faces the target), hold the middle of the cord down with your foot, and pull up on the handle of the bow with your left hand. As the bow bends, slide the string loop onto the nock with your right hand. Presto.
http://www.3riversarchery.com/Bow+Accessories+Stringers+_c46_s210_p0_thumb.html
Any of these will work fine. Just make sure it’s long enough for a longbow. Recurves run a lot shorter, and so do their stringers.
You can also string a bow with the step-through method, or by pushing the tip against the inside of your foot, but the stringer is the best and safest way, for both you and the bow, especially when you’re starting out.
There are some youtube videos that will show you how.