I wouldn't suggest Inflatable pool either since that could lead to potentially serious problems if the inflatable ring gets a hole in it.
As for small ponds for the "Aquaria friendly" benthic species - such as Bamboos, Eppies, coral cats & Horns - the polyethyolene ponds (such as Rubbermaid has) which can often been found at livestock stores - is a good choice. The best ones for small sharks or rays - range from 300-1,000 gallons.
And have the room to make nice mini-habitats for the sharks/rays.
These often cost only a couple hundred bucks - so they're much cheaper than buying a glass/acrylic tank of equal volume. And they are much better designed for sharks or rays.