Very nice build! I love compact triple lights.
You can get aluminum spacers here . For all practical purposes they will work as well as copper.
If you insist on copper you can make your own with hand tools like this . Not quite as pretty but works just as well. I bought a 12”, 3/4” copper rod about 2 years ago, used it to make 7 or 8 heat sinks (4 triples) and still have plenty left.