Theres a couple of shops in the royal plaza complex, between the Rila & Samokov Hotels, on the main drag that sell a wide range of ski boots.Thats just 2 of many.Shop around.
Fiona
i bought some boots from a shop next to Pete's bar just opposite the gondola opposite the Samakov - great price and very helpful - spoke good english aswell which helped when choosing from a big selection.