I accidentally missed one of the items that i bought and now i am missing the complete outfit. Is there a way to get it?

I don't think so

That's a pretty big mistake on the developers part.

Big mistake indeed.

Well, here's my suggestion. Make a new LBP profile on another user (or use an existing one as long as it's on a different user), collect some collect-a-bells on there, then go to the shop that you missed that prize bubble in. Once you're there, turn on a second controller and get your own user to join. Use your new user's collect-a-bells to buy the outfit that you missed the bubble from, and use the second controller that your profile is playing on to collect the bubbles spawned. Problem solved. (you can do this with friends over the web, too ;D)