I have purchased in the past from here, a German ebay site.
http://www.ebay.de/itm/Pufferpatrone...4y03XlS7aFVJBA
Sadly, when I translate this page it says they now do not ship to the USA. Doing a Google search however with the product name reveals another site:
https://colt1911.eu/epages/15e1c7b8-...ectID=16198579
It would appear that these red plastic rounds are available in Europe; I hope someone finds a place that will sell/ship to the USA (I have all I need from the ebay site from when they used to ship to the USA). These red plastic jobs cycle perfectly in all my .30 Luger pistols.
Some additional discussion
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http://luger.gunboards.com/showthrea...source-located
In the past I have also purchased dummy rounds from the link in BogeyB's post. I modified them to secure the bullet tightly by putting a little solder in then heating with a torch. Be sure to verify length is correct before you solder. Also the soldering step discolors the case but you can shine it up again with emory cloth and add a silicone 'primer' after it cools. While the rounds I purchased have a hole in the side, for safety I would suggest marking them more prominently with some kind of color. I never allow any live ammo near my workbench but from certain angles these rounds look real and that makes me a bit uneasy. One more reason I prefer the red plastic ones.