Riles, I don't own a pool but have 'retro'd' in steps to ~20 liner pools (I've also done it the way Andy said ~ 6 times). Another option is to get 'drop-in' steps - but there are drawbacks to those.

The only brand of the retro steps I know of is cardinal. Baystate Pool Supply, a subsidiary of Ft. Wayne Pools, sells them -- last year I talked to someone there and they said they don't sell to the public - I did a google search and found Cardinal, but when I called them the person I spoke to wasn't sure how to order them (they weren't on their web page). I offer this in case you may have more luck than I did last year in ordering them. If you can procure some, I'll give you some tips on installing them