So when you say "old" you don't just mean blue vs pink, you mean the older style pins on the valves, right? I check ebay for empties once in a while and those seem to be harder to come by. Most listings don't tell (or show) you which kind of valve they have. Here's a listing for four that has two cylinders with one style and two with the other.
I think there are at least three different valve types, and some of the non-sodastream-branded cylinders have the easier fill valves.