I don't think I have ever seen arm warmers that big. My first thought is to try using legwarmers. Maybe take them to a drycleaners and have them cut off and hem them above the zipper. (I learned of drycleaners as a cycling clothing modification source a couple of years ago. Bought some really excellent jerseys from a local shop on big-time sale, only to learn the zippers lasted about 2 weeks. Bought some local zippers then took the 4 jerseys and zippers to the drycleaner. For about $120 I know have 4 great jerseys. (I paid ~$30 for them initially plus ~$4 a zipper. The workmanship on the zipper replacement is very good.)
Ben