Hi,
I found >Safety 1st Swing'N Lock Gate< that says it can be tension mounted and used on iron railings effectively. Though I have never tried it.
OneStepAhead has this one: the end is bent metal to go over an iron rail.I believe they ship out of vernon hills,or somewhere up in northern IL.
http://www.onestepahead.com/product/85217/677/117.html
What about a few panels of a super gate at the very bottom of the stairs. Set it on the main floor, bend it around the base opening of the stairs, and along side of the railing, then secure to the railing with velcro straps. Beware though that less panels makes the gate wobbly. You'd just have to try it and see. TRU sells the gate, and a 2 panel extension. One step ahead has a "Configure Gate".
Good luck. Hope you find something soon. Don't forget to check ebay or craigslist, maybe you can find one local that you could pick up cheaper than new.
There is a kids sale in lake zurich coming up in two weeks I think, check on craigslist.
Melanie