This is the seriously easiest thing to make ever. It can also be made totally free.

Collect your scraps in a gallon freezer bag and store in the freezer. When it’s full make stock!

onion peels
garlic peels
celery anything. leaves of course but got a stalk or two that you just didn’t get to? toss em in!
carrot nibs and peels. 
parsley stems
and herb stems if you like it!
leek tops
spring onion roots (scallions)
shallot peels
you get the idea

When you want to make stock either place contents of your bag in your crock pot or on the stove and fill with water to top. The more water the less intense also cook time. I use a crockpot, a 5 quart and 1 gallon bag of scraps. I fill my crockpot up to the top and turn it on for 4 hours. Depending on what I’m using it for I’ll cook it just an hour or two. If I’m going to freeze it I’ll let it cook 4 hours, which makes it really intense but I don’t have a lot of room in my freezer so I consider it condensed stock. When I cook with it later I water it down.