Making your own stuff for CQ mods is hard, and swiping stuff from other people is bad juju. >:(
There is a lot of public domain stuff on the internet, but it is in a variety of styles, most of which do not match the style of Chex Quest, and if it doesn't look or sound good in Chex Quest then it is not worth using. :(
To make life easier, this thread is a list of public domain (CC0) graphics, music, and sounds that match the Chex Quest aesthetic or can be modified to do so.
Feel free to post anything else you find that is Chex Quest compatible. Make absolutely sure it is in the public domain.
SPRITES
32 Pixels Tomato Plant (Monklet80) https://opengameart.org/content/32-pixels-tomato-plants (https://opengameart.org/content/32-pixels-tomato-plants) A tomato plant with and without fruit.
TEXTURES
SKIES AND BACKGROUNDS
Sky (knik1985) https://opengameart.org/content/sky-7 (https://opengameart.org/content/sky-7) A dark stormy sky and some light puffy clouds that might look good in a skybox.
EVERYTHING ELSE
Zortch Unused Textures (mutantleg) https://opengameart.org/content/zortch-unused-textures (https://opengameart.org/content/zortch-unused-textures) Despite the title, most of these textures would be more at home in DooM than Chex Quest but many may be useful.
Zortch Unused Textures 2 (mutantleg) https://opengameart.org/content/zortch-unused-textures-2 (https://opengameart.org/content/zortch-unused-textures-2) As above these tend to be more DooMish than Chexy, but there are some nice computers.
MUSICS
MIDI
MP3
OTHER
SOUNDS
MISCELLANEUS