The problem with Formula is they make so many hubs that may or may not utilize the same or similar freehub. If your shop can't sort you out, you may end up trying a couple of freehubs which you think may be compatible before/if you are able to find the appropriate one. One idea is to try and reach out to the bike manufacturer, especially if they spec the same hub on a different model bike in their lineup.
Out of curiosity, what cassette are you hoping/wanting to run? Sunrace has recently started making cassettes at a more reasonable price for xD drivers, and e13 also have some nice and functional albeit costly xD options.