Usually compost is made from stuff that would otherwise be waste. The stuff in your compost would rot and off-gas anyways. I think intentionally composting actually results in less emissions than what would happen naturally or in a landfill. I wouldn't buy stuff just to compost it (I'd just buy compost, which, in my area, is usually made from yard waste collected by local municipalities). If you need a lot of compost you can usually intercept quite a bit of material from the normal waste streams for free. I.e. you can usually get arborists to dump tons of woodchips in your yard, talk to coffee shops to see if they'll give you their spent coffee grounds, etc.