S.M.
DON'T DO PRESSURE-MOUNTED AT THE TOP, as someone suggested.
The reason the pressure-mounted ones are not recommended for the top of the stairs is that they always have a bar across the bottom that is easy to trip over. If you stumble a little at the bottom of the stairs, no big deal, but you don't want that at the top. I was visiting someone's home who had one of those at the top of the stairs, and I stumbled and nearly tumbled down the stairs while holding my 3 month old.
If you're absolutely sure there are no studs at the top of the stair, get heavy-duty drywall anchors. I've seen some at Lowe's and Home Depot that say "Supports 500 pounds" or more. And make sure it opens over the floor, never over the stairs.
Once you figure out how to use those heavy-duty anchors, you ought to be able to use any gate that is recommended for the top of the stairs, but I have one that I particularly like. It's by evenflo and it is made of wood and is very sturdy. And it comes with matching boards to attach to the walls (if necessary) with those wall anchors (much like Tammy L recommended.) It's called the "Evenflo Home Decor wood swing gate" on the Wal-Mart website. It's $80, pricier than most, but it's attractive and sturdy and it can be taken off the hinges in-between children, in case you plan to have more later.
Just accept the fact that a hardware-mounted gate will probably be a bit of a hassle to install. I'm kinda sorta handy and it took me all afternoon. But it WILL be well worth the effort - you REALLY don't want to have visitors (or later, your toddler) tripping over the bar.