This ad literally popped up in my feed this morning:
https://ebrandon.ca/itemview.aspx?item_id=2454063
We just build our beds on the ground using cheap spruce (we keep moving/tweaking, so longevity isn't a huge issue). We've also built them out of repurposed/reclaimed wood.
I also regularly plant in 5 gallon buckets (make sure it's food-grade, with recycle triangle 5 on it) and other "found items" that hold 5 gallons of dirt or more. And yes, you do need to fertilize them regularly if you plant in containers like that, but a bottle of liquid fertilizer costs $15 and will last the whole season or more.