What is the most available high-temp superconductor for samples and study?
eBay is not the right resource for materials like this, and neither are little shops. This is a highly specialized material, which is rare and probably very expensive. If it weren't you would find it in every motor or generator. I would contact the manufacturer for information on distributors (if any) and availability.

Note: If it conducts only 40 times better than copper (website says 100) technically it's not a superconductor. Those have zero resistance.