I bought my doggy steps on EBay.  The ones in the store and advertised on the TV are usually around a foot high, so, they weren't big enough for the bed.  I was able to get a some that are a little over two feet high and they work great.  The only thing is the shipping was a little high because some of them run heavy, depending on what they are made of.  But, there's lots of variety on there.  
