They are VERY hard to get hold of, and even if you do manage to find one, you're looking at paying at least £30. Being the top seamstress that I am, I figured I would have a go at making my own. I started out with a cheap pre-loved white pocket nappy.
Great stuff. I then got some red embroidery thread, a good needle, and stitched a basic Blanket Stitch around the entire nappy. I also changed the tab snaps to red to match.
I also replaced the elastic - now if you're planning on doing this to a nappy, please replace the elastic BEFORE you begin doing the Blanket Stitch. Doing it after is a right pain in the bum. Trust me...
I was worried that there would be wicking through the stitch holes, and also that the red thread may 'bleed' through onto the nappy during washing. Shouldn't have worried. No wicking at all (and Squid has had it OTB many times in the past couple of weeks), and no bleeding of the red dye either. I'm pleased with the result, even if The Bloke did look at me strangely while I was doing it.
A Side Note - Because of the ease of converting your nappy into a 'Baseball', there are a lot of fakes floating around. The original Baseball ONLY came in v4 pockets with snaps, and retains the white snaps (this is part of the reason I chose to switch snap colour, so it could easily be spotted as a replica should I ever sell it).