Mars and some random one off amazon.
The c02 pads work pretty well actually. Better than nothing. Ive been also using something called tnb naturals the enhancer and those work alot better. Ppm around 1000 if unsealed.
Here's some trichome porn:
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Those c02 pads are just large versions of those things they put in produce to make it last longer... I'm pretty sure. I think you could get them for dirt cheap