Oo, you can buy mine for 500 Euros!
But you might find them cheaper at
OBI or Dener (if that's the name). Not sure which one, but we saw something similar in one of those places at Meglinger Str. a couple of weekends ago. I know that's a bit of a hike from you, and if I knew when we were next going I'd say we'd collect some, but I'm afraid I don't know when we might actually get there next. Of course, I guess you could make a day of it and get some fish n chips into the bargain...
Not sure about making them yourself. The key is in the spike, not the "bubble", because the spike contains an adjuster so you can regulate how much water they deliver. Other than that it is doable, we replace ours with large (1.5L) bottles when we go away to make sure the veggies still get watered.