The Nacon Asymmetric and Revolution pads are worth a look, I found the Asymmetric to be a solid piece of kit personally. Hold in mind one or two features are missing compared to the official controller such as not having motion controls, I'd definitely read up on it before buying.
There's the Hori Onyx which has a similar layout, but I'm not sure I'd bother taking a chance as I've seen a lot of negative reviews on it.
Outside of that you might be able to find a no-brand Chinese equivalent of the above for less money, but they'd be a bit of a gamble.