I overpaid for my GSG-5 mags at the big Chantilly, VA gunshow back in February. There's several thoughout the year. I'd call around to different gunstores "hi, do you stock magazines for the GSG-5? no? k, thanks" until you find one
Give shooters choice a call. I've gotten AR and AK mags there in the past. They could probably order it for you over the phone and then you could drive out to pick them up.
I stopped by Shooters Choice in Dover today, $131 later I have 4 22 round oem mags.
There is also a place up in Newark DE that had them in stock. I think it started with an X they had the no name ones and oem. $19 and $34 respectively.
There were some at Valley Forge show on Sat.I got a Umerex/walther colt M4 .22Lr and an Xtra mag was 29.00.Fun gun, 30 rd mags that run real well!A little pricey but great for teaching as all the furniture is in the right, place so new shooters/juniors can then pick up the real deal and have the muscle memory all set.Very inex to feed too!
If you ever come across one and are tempted, do NOT buy the GSG mag cinch. I bought one and it both made the mags disfunctional and I believe permanently damaged them. The follower will not move properly while cinched and appears to have permanently warped the two mags I had in there.
The link here is for ProMag's version. It seems other people have had problems with ProMag mags in the past, but for the GSG 5 version I've read good things. Best advertised price I found was $16.84. I think I've seen the GSG branded ones at a small gun show for about $25. You may want to check out the smaller gun shows that are held in VFW posts, etc. Not too many people, usually dirt cheap to get in, and from what I've seen, good prices.
Just google for "gun shows delaware", etc to find a site with a list of dates and locations.