Congratulations on your new home! Will the gate be at the top or bottom of the stairs?
Do you currently use gates anywhere in your house? We have the expandable baby gates, the kind in Magnus' first link, that keep the boys in the back of the house. I have to caution you if you are not accustomed to gates: you wouldn't believe how easy it is to forget they are there and fall over them (or maybe it is just me, lol).
I was carrying laundry and moving fast once, fell right over the gate.
Luckily the laundry cushioned my fall.
Do you want to be able to step over it or open it like a gate? All of the ones that open that I have seen have the bars fairly wide apart for small dogs.