What you're looking for is "audio samples"(individual recorded samples) or "loops" (longer recorded beats or riffs). You can find them all over the web, always try google first
You could also try different VST Plugins, which are essentially those many different instruments you see in FL. There's a lot out on the web.
These look promising:
http://www.stonewashed.net/samples.htmlhttp://www.kvraudio.com/Or you could record your own samples like a pro. But that takes the hardware and much more time.
Either way, it's gonna take some research. Quality free samples are quite hard to find. That's why I'd recommend investing in a program like Propellerhead's Reason, if you have the money.
Good luck, welcome to the wide and intimidating world of music making