I love Exhale's bags in concept.... but for the price of a single bag I can make the equivalent of dozens of them with my InstantPot, some mason jars, bit of cloth tape, and a couple bucks worth of rye grain. Granted it takes a bit of practice to be able to do things cleanly, but even a "failed" jar will still pump out CO2 (it just might stink while doing so).
A liquid culture syringe for something like Pleurotus (oyster mushrooms) is about 10-20$ on Etsy/Ebay/other websites, and will 'noc up several dozen jars if you're judicious with your dosing, AND you can even make your own LC to "refill" your syringe. Plenty of other species to choose from as well, depending on your preference.
Then again I've seen my local hydro shops selling "CO2 generators" for 50$ that are nothing more than a 2L plastic bottle with some sugar water and brewers yeast (called "The Enhancer", catchy), and you can toss that together in your kitchen in 5 minutes if you get bored.