Where do I find the enclosures for an Arduino and a bunch of sensors?
I think your problem is that you are looking specifically for Arduino enclosures. Such a search tends to lead you to boxes for just an Arduino and nothing else - maybe with the headers poking out the top.

Instead, you should be looking more generically. There are thousands and thousands of "project enclosures" from many manufacturers (Hammond is a big name in these).

You take a generic project box of the right size and shape for what you want to create and in a material suitable for your needs, and cut / grind / drill / glue it to be what you want.

Most electronics suppliers will have an "Enclosures and Housings" section on their website. Have a browse through your favourite.

As far as air vents go, one good trick I have used is to simply drill holes (with a steady hand so you get them lined up nice and straight) and glue some fine mesh on the inside.  One good source of fine mesh is to go to your local smoking supplies reseller (tobacconist) and buy some "pipe meshes" (the flat ones, not bowl shaped ones) - also available from low-cost online resellers of any old junk (eBay, etc), which look like this:



(Also available in brass).  They typically come in different mesh sizes to suit your needs.