These are what I have for the bed. They come in a four step model also:
Amazon.com: Pet Gear Easy Step III Pet Stairs,... I like these because they are light weight and easy to move for vacuuming. I did have to train her to use them. I helped her at first and treats are encouraging also. She was still quite small when we started training her to use them so I guess they looked pretty big to her.
I think you can get them for less on ebay and other sites.
Make sure you check how tall you need them to be. Some steps are only like 14" tall and are not high enough for a bed.