I’d search using different words. Maybe something to do with dorm or camping bedliners, Sleep-bag. Sometimes that will show different brands and solutions. Read the reviews too. People will often suggest a different brand or a way to do better and cheaper.
I have a fleece bag liner but rarely use it as such. It gets used on top of the mattress and under the other bedding or sleeping bag. Keeps cold from coming through mattress especially if you need to use an air mattress for a time.
My issue with the liner is the same I have with a sleeping bag. Find them confining so will use the sleeping bag like a blanket, over or under depending on temps and other bedding we have. If temps really drop will zip up and use as a bag.
It is good to use some sort of protection of the sleeping pad as many are not recommended for washing. This can be a cover if used like a blanket, a folded flannel or fleece sheet you use like a liner and slip in between.
Socks fleece or wool help in cold, fleece pants or even wool boxers. Costco sells a brand of silky like thin tops and pants. Brand is something like 32 Degrees. About now they are on sale $2 off and are like $7-9 for top.
I suggest to friends those nylon head band, pull over scarves. They call them magic scarves, face mask, helmet liner. If you buy 9 they can be found for $10. They are great around your neck sleeping or daytime to keep the chill off. Pull up or down over your ears, head, mouth. They come in strange patterns or you can find sets of solid colors. They are very light weight but hold up well in my experience. Buying as a single seems to cost as much as buying 9 no name in a bundle. I have both and can’t tell much or any difference.
We loose much of our heat through our heads so, as mentioned a hooded sweatshirt pulled tight around your head works well. A stocking cap or the above magic scarf can work too.