We never use top sheets and I have just found that you can get just fitteds in high thread count. Sheridan do a 1200 thread count fitted but is a bit pricey....David Jones stock them....Still they work out a lot cheaper than the equivalent sheet set where the flat sheet has literally never been used!!

Other brands do them as well but you have to order them...i searched a few of the sheet makers site found out who does them and then ordered them through our local bedding store that stocks that brand.