I had the same question myself and the day before I picked mine up I did some searching on this forum and found two responses that kept popping up.
The first was a username that has not posted in awhile but there was an email address floating around that may or may not still work, i did not try.
The second was
this link to Zeigler Ford... They offer some great pricing and it appears that if you purchase your extended warranty within the first year its the same price as buying it the day you buy your truck. My dealer stated that they can be purchased anytime within the 3 year factory warranty but I'm not sure if the same pricing applies.
I set a reminder for 11 months from now to check out my extended warranty options and go from there.
For those of you that already purchased extended warranties on your new trucks and are kicking yourselves after seeing the pricing at the website above they can be returned within 30 days for a full refund and a pro-rated refund after that initial 30 days.
I am not affiliated with Zeigler in anyway.