Try S&S firearms in NY state they have a big selection of antique military parts. They also make reproduction parts for some muskets. They may have a spring.

Thanks to both of you;I'll try S&S and Lodgewood on Monday

Who's the contractor/maker of the lock and musket? Strictly speaking there was no Model 1812, they were all M1795's, in various permutations as individual contractors saw fit and by the relatively loose standards adhered to by gov't inspectors.* With the M1816, parts designs and dimensions tightened up a lot. I've swapped mainsprings, tumblers, etc. around in a bunch of muskets up to M1855's. Dates on the locks don't signify model numbers.

For stuff earlier than M1816 it's best to have the parts in hand and do a close comparison. 

My apologies if you know all this. I always figure there's a newbie out there somewhere who doesn't.

*Eli Whitney, as a musket contractor, was notorious for "not following the script" as it were. I once restored an 1808 Whitney that was the primary reason for my hair turning gray and falling out.