Unfortunately, the only US source, Don Ginther of RC Ballasts, finally retired and no one else is making bulbs commercially right now. He was using a Bantock mold and I think I read somewhere he sold it to Midwest Model Yachts. You might email Brian at Midwest and see if that rumor is true and if he will do a one-off for you, or if he knows who has the mold. I had a conversation with Don right after he stopped making the US1M bulbs (I eventually bought his US1M pattern) and he said that IOM mold is a very nicely milled two piece aluminum job with notches for the brass rod and makes the bulbs in one pour.
If you don’t want to make your own bulb the way Claudio does, I make US1M ballasts for myself and a few for others in the class and could make an IOM bulb for you without the brass rod at a reasonable price if you have a full scale profile drawing or a set of dimensions to use as a starting point.
I would take the drawing and turn a hardwood plug for the master. From that I would make a cement half mold, pour the two halves and mail them to you - USPS Flat Rate (<$20). You would have to glue them together (JB Weld works) and cut your own slot for the fin. I can do the gluing and/or the milling the slot, but that increases the cost substantially due to the added labor.
My bulbs are made from scrap range lead and are not as hard and therefor difficult to cut the slot as Don’s, as he used an alloy of lead and antimony, but being softer, mine bend rather than shatter if you drop them.
Unfortunately, due to all the labor to make the plug and mold, just the halves would be about as expensive as one of Don’s finished bulbs. However, if you are interested, go onto RC Groups, find my profile (under FoamCrusher) and PM me and we can talk price to see if it is worth it for you.