Frozen or fresh. Canned have too much salt and way too many additives.
I have a neighbor that grows peas in his garden specifically for my ducks because no one in his family eats peas. In fact, no one in his family eats anything out of his garden, but he loves to garden so he basically has it all for my ducks and chickens. Saves on my feed bill lol
Check at a local Save a lot (if you have it there) or some inexpensive grocery store. Sometimes you can find bags of frozen peas for 60 cents! But as aineheartsyou said, make sure there's no additives or salt!