There are indeed dual x16 slot motherboards, but they tend to be quite expensive. In particular, the x79 chipset supporting the flagship LGA 2011 processors (Sandy Bridge-E) have more than the usual number of PCIe lanes, and tend to support this configuration.
However, an x8 slot will not handicap your performance, especially if it's on a PCIe 3.0 board. PCIe 3.0 has double the bandwidth of 2.0, so an x8 slot for PCIe 3.0 is identical in potential performance to an x16 slot in PCIe 2.0.
Even then, an x8 slot for PCIe 2.0 is more than enough for the vast majority of cards, and even the most demanding cards only see about a 1%-3% performance decrease. In general it's not worth worrying about how many PCIe lanes of what generation your board has as long as it's at least equivalent to x8 of PCIe 2.0.