You should be making your own compost every year if you have space. That way you control what goes in it. Yard waste and some kitchen waste. Make a circle or square with chicken wire and just start adding layers to it. Feed it a nitrogen source or use a compost starter like Jobes as directed. You can make a compost bin out of tons of stuff....Google it, there are tons of instructions and ideas for compost bins.....its simple.
How much compost depends on your growing mix and rainfall/temps/ect ect ect. Even type/size of pots. Plus the quality of the compost....how much freaking dirt ends up in it.
In my climate with my mix i can go with 40% pine fines, 40% good compost and 20% rice hulls. That is a local growers mix they sell to nurseries and it drains like mad. Needs almost daily watering with fabric pots in the summer. If i cut that mix with 50% ProMix i dont need to water nearly as often and it still drains good.
In my area ive had good luck with Just Natural brand mushroom compost. Some people in other areas said theirs had too much dirt/sand. What ive gotten has been REALLY good stuff for bagged compost. So good ive grown stuff in it with nothing else added.
https://www.lowes.com/pd/Just-Natural-0-75-cu-ft-Organic-Mushroom-Compost/3426612