They have some in Dunelm Mill I also have a fitted sheet for my bed which has the usual elasticated corner but the bit you lie on is fleece. I've had it for years so not sure where I got it from but it's great - totally gets rid of that horrible cold feet feeling when you first get into bed.
I would expect John Lewis. You can also put a pure woollen blanket on top of the mattress and then cover it with the sheet to give extra warmth. He might still get a very cold face though, be careful with sinusitis etc.
DS's (4) room is like an ice box during winter so I'm trying to think of ways to keep him warm - without resorting to a oil column heater!!
He is already in flannelette PJ's but with cotton sheets and summer duvet. I'm thinking of going to winter sheets come November and maybe layering another summer weight duvet inner inside his cover or replace current summer weight with winter weight.
Where can I buy flannelette sheets from? I've never seen them around, there are some on ebay but not easy to return if they are rubbish. Would it be better to get another summer weight duvet (that DD can use next year when out of the cot) or just go for the winter weight (can vac store summer weight inner).