Post a WTB ad in the forum classifieds and you'll almost certainly receive the correct grips. Some ebay sellers know their stuff, but many have no experience with firearms and are simply peddling items they stumbled across.
N frame grips are wider through the medallion, but it takes quite a bit of experience to identify them from the front without a side by side comparison (N top, K bottom right, J bottom left).
They're easy to identify, if you can see the back. Compare the length of the ledge that runs from the circle inserts (washers) to the backstrap (K on left, N on right) and notice that the ledge is almost twice as long on the N frame stocks:
Magna
Target
It's much harder to identify target grips made after 1986-87 when they dropped the circle inserts.