Look at the Intel Intel X540s (or similar with the 82598 chip). Dual RJ-45 cards are often under $100.
Here is an example:
http://www.ebay.com/itm/Supermicro-AOC-STG-i2T-Intel-X540-10gbps-Network-Interface-Card-NIC-RJ-45-
I believe the Supermicro cards aren't locked/ will work on any system, but I'm not positive. These cards are also getting a bit old.
Linux drives are available, but I would double check that there aren't any warnings related to your specific OS.
There are other 10gb RJ-45 examples, but the X540 is likely the most compatible.
A few things to consider:
Your motherboard must support SR-IOV and VT-D. Also, sometimes although the motherboard and NIC support SR-IOV it isn't implemented in the OS. ESXi is a good example of this.
10gb RJ-45 is power hungry compared to fiber or DAC versions.
I'm guessing you want RJ-45 because you already have ethernet cable in place. Note that with Cat5e or Cat6 the max cable length drops from 100m with 1gbe to 45-55m with Cat5e and Cat6. Regardless of distance unless you're using quality pre-made cables or have excellent cable runs and patch panels. Cat6A or higher is recommended and proper cabling is still important.
If you are able to move to fiber dual port Mellanox ConnectX-2 EN are around $50 and ConnectX-3 EN are around $100. SR-IOV on the ConnectX-2 doesn't seem to be as widely supported, but the ConnectX-3 is better. Mellanox is compatible with most transceivers unlike many vendors. For short runs SFP+ DACs are really cheap. For long runs SFP+ transcievers are also moderately cheap and so is OC3 fiber. Fiber is basically immune to noise/interference and doesn't conduct electricity. I've used plenum/ OFNP OM3 in many places copper can be troublesome.
