You can also produce CO2 by combining plain baking soda and vinegar. So in theory, you don't need to buy dry ice. The only thing is that when baking soda and vinegar combine, they bubble and the gas could pop the top of an airtight container off. But, CO2 is heavier than air. So, you could place the plants in a tub on some bricks or something, and put the vinegar and baking soda into the bottom of the tub. The tub would fill with CO2 and as long as there's not air disturbance, it would just stay settled in there, around the plants. You'd want to do it outside though, in case the reaction gets messy!

This would also avoid the temperature problem that dry ice might cause.

No idea if CO2 really will kill spider mites, but could be worth a shot!

Oh, and you could cover the top of the tub with something like a towel. Thick enough to avoid air disturbance and keep the CO2 trapped, not airtight so it won't pop off like a lid would!

Sweet! Its sorta making me want spider mites so I can try it lol