If you want something very cheap (but not matching your choice of what I call candystripe) then you could try Bargaintown. For €99.00 they are carpeting stairs. Probably won't last long though.
I got one or two prices recently and they were between €800 to €1,000 for stairs only but it was going the full width of the stair. IMHO the option of paintaing either side is too much hard work with regard to painting and hoovering.
This previous thread might of be interest.
Ulster, Navan or others don't appear to do this type of carpet.