I've never shelled peas (we tend to just eat a *lot* of snap beans all summer, instead) but I can tell you that my local produce stand does (while we're at the height of the season, anyway) sell a limited number of their peas fresh and pre-shelled? So you might be able to find some, if you get lucky - probably not at a farmers' market, but maybe at a semi-permanent farm stand a little bit out in the country.