There are 6 or 7 stores online that advertise very heavily - Amazon, A Beautiful Bed, Lingerie Shopping, Satin Bedding, Comfort House, Home Visions, Domestic Bin, Laurens Linens, etc. I can't follow up on all of them for you, but the ones below seem the most reputable and / or I have heard good things about them from visitors here:
I've also heard from a lot of people asking about offers they have seen on ebay. I have my doubts - even if they are real woven polyester satin, they are probably thin and will not last as long as something you buy from a store with a reputation to protect.
Satinbedding.com - For top-end quality, these are the guys to go to. They have a really wide selection of fabrics for their satin bed linens (sheets, satin comforters, etc). As best I can tell, most of their satin sheets are made of polyester with about 4 quality levels. The "Charmeuse" line is real silk, and they make velvet and faux-fur sheets too. They also have some beautiful satin lingerie, and even sell bolts of satin fabric so you can make matching curtains or drapes. I've traded emails with David from this this company and they seem very honest.
Amazon - A number of stores use the Amazon platform to sell online.
- The sheets offered by Target through Amazon (online only, not in their stores) appear to be a decent quality - a bit thin but a decent weave. They may not hold up well to regular use, but they would be good for a few special occasions per year.
- There are a number of other companies selling sheets through the Amazon front end. If you read carefully through the buyer's reviews, you'll see that the majority of them are not well made (sizing), have colorfast problems, or of non-standard materials that don't give you the true satin sensation. Buy with caution.