Have you tried googling the data source you want + JSON? Seems like there's a lot of developers discussing the path to get that data, but I'm not seeing those links publicized on many websites. Maybe Brendan Lee has a better method?
you can easily find them by searching for
<appname> json rest api
like "twitter json rest api" or "fedex json rest api". however, these APIs in some cases are more complex than just getting a json with GET method