TL;DR: Reptile terrarium heaters, lights, thermostats, and misters might be a good source of low cost components for building small hydro & aero systems and for heating germination trays.
I’ve been shopping for parts to heat a seedling dome to the right temperature for propagating african violet cuttings. I’ve found that many of the heat mats and thermostats designed for seedling trays are relatively large–good for standard sized seedling trays for a commercial greenhouse or big garden. I also came across heaters, thermostats, lights, misters, etc. that are designed for keeping pet lizards in terrariums. There’s a lot of functional overlap between greenhouse equipment and reptile equipment, but the reptile stuff seems to be smaller and, in some cases, less expensive. I’m not sure about quality and reliability.
Here are a few interesting items I found on Amazon:
Exo Terra Monsoon Solo High Pressure Misting System, $75. This might be suitable for making a small aero system–it’s got a high pressure pump, a programmable interval timer, and a small water tank. There’s also a larger version.
iPower 6"x 8" 8 Watt Terrarium Reptile Heating Pad with Digital Thermostat, $30. This might be useful for heating a small seedling germinating tray. For comparison, iPower also has a 10" x 20.5" 17.5 Watt heating pad with a similar thermostat for $27 that’s intended for standard seedling trays. The reptile version is interesting because it comes in a smaller size.
What I like about the reptile stuff, and the reason I mention it, is that it might be good for people who have a small budget and a small space. Besides ordering online, these types of items might be available inexpensively in local pet shops.