I got a massive industrial sized bag of coarse verm for cheap off amazon. People say medium is best, but coarse works fine for me. Here's the link.. I haven't even scratched the surface of this bag, it'll probably last me years. If you want it finer it's not like you can't toss it in a blender or mortar.
I feel like a dumb ass $40 actually EasyGoProducts Palmetto Vermiculite Coarse 120 Quarts - 4 Cubic Foot of Organic Planting Soil Additive – Coarse Grade, Model Number: Vermiculite4-1 https://www.amazon.com/dp/B085G5P27M/ref=cm_sw_r_cp_api_i_C.okFbYBTR6T4
Maybe look for bulk suppliers of vermiculite or ask a local nursery if you haven't already. If you get it in small amounts from Home Depot or Lowes, you're going to pay. But I was able to get Palmetto for less than listed on amazon through a small nearby place who carried it.