There are a wide variety of options for finding the gifts for your friend's birthday parties.
Generally you would like to follow three guidelines for determining what you would like to get.
First, you want to establish a budget for your gift. Generally this would be establishing a price range. This should take into account how much you think that others will spend, your own income to determine expectations and the relative depth of your personal relationship.
Second, you would want to try to personalize a gift while also being useful. Buying something that would either be returned or thrown away doesn't do anything for either of you and a personalized gift is more likely to be treasured.
Third, you would generally want to get something a price lower than is generally available to people. This means that people will think that you spent more than you actually did. What they don't know won't hurt them.
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.