I sold my Jazzmaster to my mate with 2 of the screws for each pickup missing (he knew this before buying it, so I didn't rip him off), he went to B&Q (or similar) and they had nothing suitable. On investigation it seems like they are an unusual size and type, meaning they're not readily available.
I've had the same issue. I've never been able to source them here in the UK. You can probably snag 'em in the US. I balk at buying them online for £12 for a little pack of screws that you can probably buy elsewhere for a few dollars for a whole pack. Being weird about spending a few quid may seem silly with the amount of money I've spent on gear over the years. It'sthe price gouging element that doesn't sit well with me.
That's why my red JM only has 2 screws per pickup too.
Fender part no. 0016295 appears to be a #3 x 1" nickel plated round head wood screw also used on Telecaster...but like you say, tracking them down isn't easy.