Hi cbcb,
In most cases a fitted sheet will fit a “range” of mattress thicknesses and as long as your mattress is inside the range for the sheets you are buying (and in most cases a regular rather than a deep drop would fit your mattress) you will likely be fine.
There is much more information and some sources about sheets and different materials in post #7 here and hopefully some of the members here will be able to share their sources for cotton sheets they like (I don’t have the time/ability to keep track of pricing for sheets and bedding).
Phoenix