I want to create my own Runescape server for me and my friends to play on. I've looked at a couple of sites an I haven't found any downloadable source code. Where can I find the source code for the server and where can I find it for the client? Is it even possible to make your own private server without doing all the decompiling yourself?
It is impossible to host a Runescape private server, because the source code is not (nor does Jagex have any reason to make it) publically available, for either client or server. (And in fact, trying to do so would go against the Jagex TOS)
Runescape generates revenue from monthly subscriptions, so there's little reason to think that "private" servers not hosted by Jagex will ever be available, and deassembling the source code is all kinds of shady legal, so I'd advise against trying to do so.
If you want to play with your friends, you'll just have to do it on Jagex's own servers.