I believe the Rasp Pi cluster is one of the cheapest multi node / MPI machines you can buy. That's useful even if it is t fast. You need to practice the programming interfaces, not necessarily make a fast computer.
However, NUMA is also a big deal. The various AMD Threadrippers with multi-die memory controllers are better on this regards. Maybe the aging Threadrippers 1950x, yes it's much slower than modern chips but the NUMA issues are exaggerated (especially poor) on this old architecture.
That exaggerates the effects of good NUMA and now you as a programmer can get more NUMA skills.
Of course, the best plan is to spend $20,000,000++ on your own custom NUMA nodes cluster out of EPYCs or something.
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But no. The best supercomputers are your local supercomputers that you should rent some time from. You need a local box to see various issues and learn to practice programming.