CRS260

If the plunger tube is moving (instead of the plunger rod) making this part is going to be more difficult. As a moving part, it probably has more definite size constraints.

I would probably build a new part out of wood since I don't have plastic material thick enough to compress the spring safely. It would be a thick donut shape with two nubs coming off the sides for the screw. Alternatively, you can try to find a steel washer that is the right size. Put two holes right through the washer on either side and just screw through those.

I can say for sure it IS necessary, as it is the piece that the spring pushes against in order to drive the plunger rod down the tube.

Is that all it does? It should be very easy to replace then. It looks like that part has its own recess in the shell that it fits into. If you can cut a piece of wood or plastic to fit that recess you probably won't even have to bother with making screw holes and tapping to attach to the plunger tube. Then just put a hole through the wood or plastic that is just small enough to prevent the spring from sliding through (but allows the plunger rod to slide through of course).

It's essentially just a chunk of plastic with a specific sized hole going through it.

It probably wouldn't be very hard to create a replacement part yourself; that part looks pretty simple based on the picture. The hardest thing is getting dimensions for the part since you probably didn't measure it before it got lost. Your best bet is to find someone else who owns a Stampede (either here or locally) to provide you with the part dimensions. If that doesn't work out, get the dimensions of what the part was originally attached to (and how it fits in with everything else) and work off that.

You should check if that part is even necessary to operate the gun. It's possible that the part is a redundant safety mechanism that you won't need like the magazine-lock or the boltsled-lock on the Longshot.

From what I've seen, many people obtain their springs from cannibalizing other objects instead of buying them directly. Things like ball point pens, cheap toys, and other Nerf guns would be a good source for tiny springs. Larger springs are mostly bought or obtained from other Nerf guns. McMaster-Carr is a great source for any mechanical supplies or tools you need, but I don't know if they ship to Germany.

For your catch spring, I would find two other similarly sized springs and combine all three together onto the catch plate instead of looking for a single, stronger spring. Just make sure they are all around the same length and diameter; it doesn't need to be very precise to work correctly.

What blaster are you working on; that would help us figure out what springs you need.