If you're talking about a cup foot for a longarm - if your machine company doesn't make one, it's possible to make one yourself. My husband made one for me, using one of the feet from my machine (I think the 1/4 inch foot) and the clear plastic lid from a health and beauty product. He used an expoxy to attach the clear plastic lid (trimmed to a small dome).
We spent quite a bit of time browsing the health and beauty section looking for the right lid.... got lots of strange looks as we were calling back and forth to each other during the hunt!