You can find plenty of used, decent ones, all over the net. It really comes down to two things for you; do you want mapping capabilities (more expensive), and how small do you want the unit to be?
I have used Garmin products exclusively-a Foretrex 101 and 201, an Etrex (yellow model), a couple other mapping Garmins (dont recall the models), a Garmin Quest (I think it is called) for my Jeep, and my current GPS60CX. All in all, Garmin is easy to figure out, and once you know how yo use one model, you can figure out them all.
For convenience, I used the 201 in the Army-it only gives tracking & grids, but thats all I really needed. My next one was the Etrex, which essentially did the same thing, but in a handheld unit (I think some of the models now you can load maps into, but I am not sure). From there, I went to a few of their first rudimentary units that are no longer available, and finally settled on my GPS60-have been happy with it for a few years now.
My suggestion for you would be either the Foretrex or Etrex models-both could be found new for less than $100. In addition to a map, compass, and altimeter, you will be good to go.
Dont fall for the "electronic compass" features on ANY models; they are only moderately accurate at best, and increase the price by roughly $100, when a REAL compass, that works reliably, would cost you $15, if you dont already have one. Most model GPS's offer one with, and one without this feature-choose without.