Loops are a great way to speed up the process of creating your own tunes, which is why many people build up huge libraries of their favourites.
If you can't quite find the loop you need, though, it may not be necessary to download even more. Grab a copy of LoopJam, instead, and it can remix and rework your existing WAV loops at a click.
Launch the program and it'll immediately start playing a demo loop. The program splits this up into sections, and can apply various effects to each one. You can try to analyse this in detail, and manipulate the loop in a few simple ways, but it's much easier to click the "Jam" button, which automatically applies a different set of effects and begins to play the results.
Using this kind of random approach means the results probably won't be an improvement, of course. In fact, they're usually much worse. But as you get to hear the remixed loop immediately, it'll only take a second or two before you realise the truth, so if it's not worked you can just click "Jam" again to have another go.
If you get bored with the demo loop, then the program has several others you can try.
And if it does come up with something you like, then at any point you can click "Export" to save your remix as a WAV file.
While this is all rather basic, it's fun and extremely easy to use, definitely worth a look if you'd like to bring new life into your loop library. And the future looks promising, too, with the upcoming LoopJam 2.0 scheduled to support automatic beat detection and the ability to change playback speed, amongst other goodies.