mnw42,
38 Super mags will work just fine.
steelyblue,
Try Metalform. They're pricey buying onesy and twosies, but if you can get in on a group deal here, they're much more attractive price-wise. Metalform 9mm mags use a spacer in the rear of the mag body to fill in the gap not needed with the shorter round.
Springfields are another good option. The Springfield mag dimples the front of the mag body to accomplish the same goal. 6 of one, half a dozen of the other.
Price-wise, if you can jump in on a group deal, or organize a group buy yourself, the Metalforms become a great buy.
Or you can always simply buy 38 Super mags, that way when you fit your 38 Super barrel, you won't have to buy another set of magazines.