In need of GPS Units!

Hello,
I searched around and could not find any information on this. I am celebrating my 30th birthday this July and my wife and I want to do something different.
We want to get all my family and friends together and to do a multi-cache hunt. We would pick the locations and the cache. Then we would break everyone into teams and have them go hunting. This would be at a central location, like a big park that we could hide the caches well spread out.
Our dilema is that we don’t know where we can get GPS units. I see that their is a company for corporate outings, but this isn’t that fancy or large scale. We want to get our family and friends interested in Geocaching, so most of them do not have units.
We thought we could get the units and preprogram them ahead of time with the multicache coordinates.
Please help! Have you done this before or know where we can get the units (rent them)?

Thanks!
Jeff
Germantown, WI