McMaster-Carr doesn't appear to have 7/8 flex conduit or connectors.
This leaves me to suspect that no such thing exists. Is 7/8 the nominal (trade) size of the connector, or did you measure the nipple/fitting for the valve? If you need a fitting for 7/8 pipe, nothing about it is actually 7/8 in size.
The products sold go by the trade size designation mostly, not actual thread OD dimensions, specially pipe thread sizes. The main variation in sizing comes into play with refrigeration tubing sold by OD, and same tubing sold by ID for plumbing service. Only difference is refigeration tube has had the stearic acid lube washed out and the inside of the tube filled with dry nitrogen then plugged under slight pressure.