See my last paragraph for ordering information. Today, I ordered these inexpensive piston/pump heads. Beforehand, I opened up my Aspen to measure the pump piston (gun now back together). The third-stage metal piston measured about 5.85mm in diameter at its widest (wider o-ring seals center the piston in the pump tube to eliminate metal to metal contact). I didn't bother to measure the inside diameter of the Aspen pump tube because I assume it is 6mm i.d. I failed to measure the threading that attaches piston to rod, but my eyeball says the threading will agree with the seller's description of the pistons. I suspect this piston size/style is common in many different pumps. I ordered 5 pump (piston) heads: Four pistons from one seller have a spring, like the original in my Seneca Aspen. The fifth was from a different seller, and deletes the spring assembly, but has similar seals. Total cost was $10.00 USD for all five together.
It will take four weeks for the pistons to reach me. I'll report my results after installing same. I have taken some photos towards writing a tutorial. Soon, I hope.
There are many online sellers of these pistons. "Search "piston stage" or "pump stage". It worked for me! If you're in a hurry, Ebay lists a bunch of sellers. If you don't mind waiting for a slow boat from China, there are many sellers at Aliexpress. NOTE: Some sellers featured photos of three, four, or more pistons together, but the fine print clarified that you were buying just one or two pistons for the listed price.