Where is the best place to buy spark plugs for my 1915 T?
Depends on what type you're looking for. If you want show plugs (which also work well) then get a set of straight side Champion X, shown first two on the left. The leftmost one has no patent dates and is believed to be the earliest, used from 1911 up to about 1915. Next one was used 1915-about 1921. These two are only available used unless you find some NOS. Next is the ribbed Champion, used about 1922 on. The brass top came along around '23 and the Ford script was added '23-'24, I believe. The last plug is a modern A-25 which are still available new, along with the brass top Champion X ribbed plug. These last you can get at your local O'Reillys or AutoZone, or through the T vendors.
And the new X's cost $20 each or more. I much prefer buying the originals at swap meets or estate auctions for $10 or less (sometimes a LOT less).
I should have mentioned that you also often find them on eBay.
All the dealers carry plugs