Until recently, I noticed that they were available on eBay, at least from two suppliers, both were in US. Three different sizes of sac were available. It took me a while to sort out the right size that suits my pens by searching information on the Internet. What I need was "standard" size. I finally bought several from one of the suppliers. The sacs were not expensive but comparatively the postal fee was really something.
The sacs arrived in two weeks from the time of the order. Each sac was provided with a little plastic ball (the supplier called it "retention ball") to stuck the sac to the plunger of the pump.
The first and the most difficult step of the repair was to take the pump out of the pen barrel. It normally requires a special tool but I did not have one. I risked my own way by wrapping the outer screw segment with soft material and then twisted it out with pliers. The broken sac was then removed.
The new sac was longer than required. It needed to be cut down to a suitable length. I cut it down to a length of around 27 mm as an informative website suggested.
The tip of the sac had to be inserted into a little hole in the plastic plunger. Then, the "retention ball" was put in to permanently stuck the tip in that hole. A special tool is normally required. I used my own way, of course ─ I firstly put the retention ball into the tip, laid the combo flat on the desk top, then I positioned the plunger hole above the ball-filled tip and pressed it down so that the tip (with the ball) fitted into the hole.
The next step is to fold part of the sac inside-out to the extent that its end can affixed to the metal die of the pump. I used the end part of a ball pen to help folding the sac inside-out. Then, I affixed the folded-up end of the sac to the metal die with all-purpose glue. The pump was then screwed into position.
The final step was to check that the barrel end was air-tight. If so, the repair was completed.
