Josie...Sorry more people haven't responded to your post to give you some ideas. Here are my thoughts...

1) If you aren't opposed to buying something used check out Craigslist.com for walk behind edgers (they have two wheels in the back & one in front where the blade rotates). I use a stick/hand held edger & I tell you, it's the one thing that really, really makes my back hurt to use for any length of time (I'm tall & have to bend over slightly to use it). If I were doing it over I'd definitely get a walk behind machine where I could just push it along as I edged the grass. Mclane is the brand to look for & anything with a Honda engine would be the bomb. I would have no problem buying a used edger if it starts easily & looks like it's been cared for (i.e. no rust or missing pieces of any kind).
2) If you can't find something you like on Craigslist or don't want to buy it used then your best bet is probably something from a Home Depot or Lowes. Again, I'd go for a walk behind over the stick version since he's already got a trimmer.

I'm no expert & you've got to be a little careful about buying used tools, but the absolute first thing I would do is check Craigslist for a used Mclane walk behind edger in good shape. And if it had a Honda engine I'd be on it like flies on a rib roast :). Anyway, good luck with your purchase. Damn, I wish my wife would buy me tools for Father's Day....