How do I get the RAP of a player with scripts?

I'm making a game where I want players' RAP of their collectables above their head, how do I get the total amount of their RAP though? There's no function I can see to do this?

You'd need to use HttpService to calculate a user's RAP. Since there's no web API endpoint to get a user's RAP, you could iterate through all items a user owns, looking for limiteds, and getting their RAP to add to a total. You'll need to proxy HTTP requests since they cannot be sent directly to the Roblox website with HttpService. See here:

http://wiki.roblox.com/index.php?title=API:Class/HttpService