There are three separate approaches that you can take to et the theme that you want for your birthday party.
First, there is going to a store and simply buying what you need. Different parts of the country have different providers including Party City, iParty, etc. and even Walmart can serve for your needs.
Second, visiting online stores and having them ship you the supplies that you need can help you acquire what you need.
Third, you could simply hire a company or planner to do this on your behalf. Simply look locally for an event planner. These people are generally more expensive than doing it yourself but for some the savings in time and the professional approach is more important.
Any of these approaches are great, but it's most important that you make sure that you get what you would like and enjoy while staying within your budget limitations.
Our company, THEY improv, provides a variety of entertainment options that can use your theme and provide you with appropriate entertainment within that theme.