There are various ones on eBay that pop up cougar, torino , mustang pads are the same for 1968. The wood grain is hard to find though. I ended up getting an amazing restored Deluxe Steering Wheel and Pad with the Wood Grain from
http://www.bestinshowsteeringwheels.com/ They specialize in Mustangs.
Not only did they locate the wheel, pad and wood insert and then did a full restoration and when they were done they didn't like the outcome or the wood grain so they located and restored a second one. Below is an example of their work: