Rubbermaid is a safe and cheap way to keep fish.. they are approved to hold animals and they don't leech out chemicals (most have to be graded for food safe).
Sterilite is used (Walmart brand), but is a bit more prone to crack.. Rubbermaid doesn't as easily. Just make sure not to fill it too full of water is all.
MANY breeders and people who keep a lot of bettas use them - cheap and safe to use. If they weren't safe, they wouldn't be used in the numbers they are. Have yet to ever hear of a death due to any chemicals being leeched - the only dangers is people using soap to clean them and leaving residue, or had stored items in there that if not washed properly, could be a danger.
But a brand new tub cleaned out real well with clean water is safe.
(Granted, like all containers AND tanks, something could of gone a bit awry at the processing plant and something could of been sprayed into the tank/tub..)