We were faced with the same situation when we bought our house. FWIW, you need to enclose the stairwell to meet code so it may ease your resale or if you are renting it maybe a stick you can use to get the landlord to fence it.
We actually bought fencing from Home Depot for fairly cheap in the style of wrought iron but its actually painted aluminium. http://www.homedepot.com/p/45-in-x-30-in-Empire-Fence-Gat...
You can find it in the gardening section. I was actually more worried about my daughter plunging over the side of the wall which is about a 12 foot fall onto concrete. This fencing took care of that.
It's very easy to install, very sturdy, attractive and in lengths that worked for the length and width of our stairwell. We've had it for 4 years now and just had to paint one spire so its held up very well.