Here's an example of VLF microphones, although I can't find pricing: (kinda want to buy one now to play with) https://www.grasacoustics.com/products/special-mi

Here's an example of VLF microphones, although I can't find pricing: (kinda want to buy one now to play with) https://www.grasacoustics.com/products/special-microphone/infra-sound-microphones https://www.grasacoustics.com/products/special-microphone/in... Excerpt: "To achieve a very low low-frequency cut-off of down to 0.09 Hz, a special microphone combined with a special preamplifier and a low-frequency adapter is used. To account for pressure variations close to 0 Hz, a special ambient pressure equalization system with a very long settling time is used."