I bought sheets at target for my kids' beds (which have thick memory foam toppers on them) and they are really nice and deep. They have a double elastic on every corner which seems to make it easier to put on plus keeps it on. They aren't cheap though (IMO-maybe I'm a cheapskate) so right now each kid has one set and for washing, we take them off early and get them all washed and dried so they can put them back on at bedtime.
the bonus-no folding of sheets.