i would think that you could make just about any plastic bottle fit with some ingenuity. I saw the powder hopper the guy on fleabay is offering. You can always go that route if you fail miserably at the plastic bottle challenge.
A bottle that might work is one from a kitchen sink soap dispenser. Not sure how the original bottle attached. But if it is just a snug fit, I think finding something you can make fit will be pretty easy. Other plastic bottles with smaller necks than soda bottles that come to mind are fuel and oil additives, hand cream, and similar. Some are opaque, some clear and some translucent.
Another option is to solder metal cap off a container to the top of the measure and then use a standard soda bottle. Gee, that just made me realize that an old round metal gunpowder can would work. Solder the cap to the measure and then just cut the bottom out of the can. Suspect the neck thread would be the same as a soda bottle in case you wanted clear.
UPDATE: I just looked at the photos for that replacement container on fleabay. I see that the original was held with a set screw. Looking at that photo, I'm willing to bet that a standard soda bottle or round gunpowder metal can will go into the measure and the screw will engage the thread.
AND YET ANOTHER UPDATE: I just had another synapse fire. If it were me, I would prefer more of a cylindrical hopper for the powder. The reason being I could then make a baffle to assure consistent drop every time. That tapered container on fleabay may or may not lend itself to a baffle. So, here is my thought. Get a piece of 2" PVC pipe and a 2" end cap. The end caps are cupped inside so you'll get directed flow into the measure. I have a lathe, so the next part is a snap for me. But you could do this with a drill press. Find a piece of PVC pipe that will fit in the neck of the measure to become the neck of the hopper. You might find that a PVC fitting is a better fit. You may have to trim whichever once is closest. Then drill a mating hole in the cap and glue that hopper neck into the cap so that it is just below the surface on the inside of the cap. All that is left is to make a baffle for that hopper and you are good to go. I have PVC pipe in the basement from previous house projects and remodels. Not sure if you are so equipped. If you have to go out and buy the 2" PVC, this is probably not a practical solution. But there is still the mooching pipe from friends option.
