I was going to suggest E-Bay and/or Amazon, too. That's where I purchased the ones for my bedroom. The bed sits high, with the surface at 26" from the floor. Lots of the newer mattress sets are even higher ... I purchased a 5-step staircase that's covered with carpet, which helps them get a better grip when they go tearing up/down the stairs. This sets the top stair at the top of the mattress, so they just step over.
I put the stairs parallel to the footboard rather than perpendicular; it takes up less space that way and is actually a straight shot from the doorway.
I wish I could remember who the vendor was; he makes them up for you as they're ordered. He has several levels of "quality", which has nothing to do with the base staircase, just the way it's finished (grade of carpet, etc.). His economy level is covered with the kind of carpet used in cars: durable and inexpensive (which is what I ordered).
His stairs also come with different tread widths so they can be ordered for elderly big dogs, or tiny little balls of dynamite. I ordered the ones for little dogs; the treads are 5"D x 10"W with a riser of 4.5". With the sides, it's 12"W total. It is absolutely perfect for little dogs, like it was measured for their stride. And because it was designed for little dogs, the overall measurements are smaller, so it takes up less space than stairs that were designed to allow a Rottie to get to the bed
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I think they were about $60 at the time, with $20 S/H. Good luck with your hunting, but you may wish to keep the dimensions in mind so you don't get one that's too big - and spend $$ you don't need to.