Re: Is a shear pin from the hardware store adequate for the prop shear pin?
If the cost is that big a deal, try going to your local hardware store or autoparts store and see if they can order you a piece of SS in the right size?
NAPA Auto Parts here stocks a good assortment of both shear pins and key stock, I've been able to get both on several occasions.
You may even try another dealer, even another brand dealer may stock those just to have on hand for repairs.
I got my last ones from a local Yamaha dealer here that stocks parts for just about eveything. They were well under a dollar each and they were OEM packaged. I bought a dozen or so, along with matching cotter pins for like $12 and change. I think they were a Mercury dealer at one point too, but they keep a pretty good selection of all the popular brands since they do service all makes.