You can buy plastic tubing from Graingers. They have it in various diameters on 1000 foot rolls. You can get it cut in smaller lengths from stamets, but it's spendy.

If you grow oysters in a tub, make sure you give them lots of air exchange and light. Oysters need both. Also, if you grow them in trays or in a tub, build a rack of some kind to put on top of the straw with some rocks or other weight to hold it down. You'll get better performance from straw if you colonize it with slight compression.

You can also save old bread wrappers to use, but be careful packing them because of the thin material. Food saver type bags also work well, as will the largest zip locs.
RR