This is one of the most useful pieces of equipment you can get for free.
Where to find them:
Go to any tire shop and ask if they have any scrap car inner tubes.
If desperate you can buy them. Oh, don't forget mountain bike innertubes for smaller jobs.
Cut them up with scissors into “rubber bands”, or “tubes”. Rinse and ta-da it is complete.
Holding hoses etc. in place while cave diving, securing a flash to your model, part of your wrench setup, keeping your suit over your boots, creating a semi-dry container for carbide etc., the list goes on and on.