Think I would be looking for a sewing machine shop that did understand what are quite 'basic' bits
I have great luck at my local hardware stores. If possible, take a part with you so they can help find the appropriate size. The prices at the hardware store will be much less than at the sewing store.
I am not a quilter but from your description I believe you are talking about 'wing nuts' or 'butterfly nuts' as previously suggested. If you could perhaps upload a photograph of the end piece of the roller, that would help to clarify the issue.
I think they are called wing nuts? I just bought some to fix a dress form I got for free awhile back.
They might be butterfly screws. Good luck with it all, love Chris
Try searching, quilters rack, parts, you might get the answer that you seek!! Good Luck