Looks like the Line 6 Mobile Keys keyboards use the same cable as the Sonic Port. Lucky for me, I have a Mobile Keys 49 and was able to use it for my Sonic Port.
I got this reply from Greg Hionis at Full Compass regarding lightning-connector Sonic Port replacement cables:
This part number is available to order from us. however line 6 is trying to get a lot of money for this cable and it would take around 3 weeks to get to you here in the states..
Part# 21-34-0240 (CABLE,LIGHTNING SONICPORT) $34.00 each + shipping
If you would like to place an order, please have the part number ready and give our sales team
a call at 800-356-5844 (please ask for sales)
let me know if you have any questions
And this reply about 30-pin cables (i.e. iphone 4 & earlier):
Yes! this 30pin cable is also available to order from us. this is a little cheaper and we are actually expecting to get these in today..
Part # 21-34-0230 (CABLE,MOBILE KEYS/SONIC 30PIN) $25.75 each + shipping
These pictures will be available on our website soon, please let the forums know!
If you would like to place an order, please have the part number ready and give our sales team
a call at 800-356-5844 (please ask for sales)
let me know if you have any questions