Retro arms shells are top of the line and very expensive - as per @Alimcd advice above, I usually pick up shells on eBay auctions...
Just this month I won an auction for a CA shell (which are some of my favs) for about 17 quid, and I watched a G&P shell go for not much more a day later.
Best way to track availability is do a search, select auction only and then order by ending soonest - may need a bit of patience over a week or two but decent GB shells on eBay are really not that uncommon ?
Edit*
Hello there! I mean i would nab that... though I'm stocked up on shells for the time being
Description says MP5 but that's just the selector plate which you can switch out with the one you already have on the cracked shell.
FYI these were part of the most expensive gearboxes available during the mid/late 2000s - quality should be fine as long as it's not actually knackered; description says never used.