Can't be bought, it's proprietary, just a molding around a regular plug though. You can either play around with existing plugs and try to hack it to fit. Or, I highly recommend chopping that jack out and replacing it with a 4 pin mini XLR, then wiring the headphones up balanced.
The plug is made of two parts. Mini jack - which is a just standard mini jack plug, nothing special here and a plastic part in which mini jack is molded. That second part is a problem here. The option will be to kick out original socket (in left cup) and replace it by minixlr or somehow take it from original cable then. I have totally no need to use balanced headphones at all and I rather prefer to not touch original socket in the cup so if I really cannot buy it even directly from Shure I will rather do experimenting with using a plug from original cable.
I know it has been a month since you last posted, but I wanted to second removing the jack and replacing with mini xlr. If you don't want to go balanced, you could go 3-pin. Possibly a more secure option than stock, with the bonus of being able to use aftermarket K702 cables (or to sell unwanted cables you make to K702 owners). I modified my SRH840's with a 3-pin after failing at finding a suitable plug for the stock socket. I tried numerous times to replicate one with various materials, but none were satisfactory. Isn't hard at all if you are careful and have a good rotary tool. Have fun!
Lunashops sells spare cables that are not as expensive as Shure's and that fit. They are much shorter though, and sleeved instead of being rubberized.
Also, the cable can't be "locked" in the headphone like the original, even though it won't unplug on it's own.
Here's a comparison between the original and the replacement: