The dealers, especially the parts guys, know less about the Venture than anyone on this board.
They were forever asking me, "Is that the 1600 cc or the 1700 cc version?" or "Why do you think you need 4 spark plugs?"
Chances are they have the bolt or a suitable sub in stock, but are just to ignorant to know they have it.
Best option is just what has been suggested here, take one of the bolts from the plastic cover around the ingition or one from the side cover and go to ACE, Lowes or Home Depot and match it up as best you can, use blue locktit when you install it and carry a spare.