Hi jasonsmith,
I don’t know the “best” place because sheets and bedding isn’t my area of expertise (and hopefully some of the other members will see your post and chime in as well) but I did do a google search on “full XL sheets” and there were quite a number of Full XL results that showed up including some that were about a 50/50 blend of cotton poly in shallower pocket depths (I would avoid 100% polyester sheets).
Some of the stores that showed up on my search that seem to have them include …
http://www.sears.com/shc/s/t_10153_12605?tName=full-xl-sheet-set.html
http://www.bestbedding.com/full-xl-sheet-sets
Post #7 here and the other posts and topics and websites it links to also has more information about different fabrics and sheets that may be helpful.
Phoenix