Actually Ralph Lauren does sell cravats, at least intermittently. I bought three woolen cravats while visiting the Beverly Hills Polo Shop in the company of Andy Gilchrist a couple of years ago. I find them of limited utility, but if I lived in a colder clime I would doubtless wear them more. They are not nearly as pleasant to wear as silk ascots though.
In the UK, you can try
www.countryties.com. They have a fairly good line of silk-fronted, cotton-backed cravats with game bird themes as well as more conventional paisleys and polka dots. They are quite reasonably priced compared to a lot of pure silk cravats. I just received my third cravat from them two days ago.
In the States my favorite supplier is
www.beautiesltd.com. Their primary thing is bowties, but they will make cravats out of nearly any of the fabrics in their extensive line. I have 14 cravats from them. A minor virtue of Beau Ties Ltd. is that their cravats are sized, which is especially nice if you have a fairly thick neck.
I won't claim either is the best, but I have found the products of both very satisfactory. I have a couple of very fine silk ascots/cravats I bought at Carroll & Co. in Beverly Hills, but when they kicked the price up to $115, I decided to forgo further purchases of such there.
Both Brooks Brothers and J. Press have small selections of ascots/cravats that can be ordered on-line.