Beau Ties in Vermont is a known quantity; classic materials and styles, handmade and very reasonably priced ($33 and up, check this one and a perfect Churchill styled polka dot). They also offer a service to convert your necktie into a bow tie.
I had an old Ralphy navy and white woven number that I loved but that hadn't recovered from a soup accident. The cost to convert is $41 including postage; only took them 10 days to turn around after they received the tie and called for a quick phone consult. You can choose the width/shape. Not going to go on a converting spree but damn loved that tie... and can now love it again.