MantisPlace sells the containers already setup for mantises, lots of various sizes/shapes.
Someone else gave the original link to a company, Neatly Smart, that sells the old style candy/gumball plastic containers (and much more). Here is a direct link to the
small container and the
large container - they cost only a few bucks each and $5 shipping. This container seems to be a standard one in the hobby besides the deli cups. Of course you have to diy to get them mantis ready.
I know lately my local
Dollar Tree has had 32 ounce deli containers (3 for $1), round 2 quart, and round 3 quart containers (and more) - all for $1 and they work great depending on size needs. I've been using them for my fruit fly cultures and mantis ooth incubating containers.
Or as mentioned by others above any empty plastic food container is great. My most recent habitat is made from a
plastic file folder box. Doesn't really matter if it is transparent, frosted, colored, etc - as you'll cut holes into anyway for mesh screen.
You'll be amazed at what you can find walking around at a grocery or department store looking at the stuff for sale simply as it's use as a mantis home.