Its not an option with Scion unless its a Special Edition (if there really is one). Although some Toyota dealerships will install leather seats for you at a price if you got the cash.
I would suggest going to a private party to get leather interior done and get some referrals before you go get it done.
I dont know how much they would be....I would guess the price varies depends on who you go to. Just my two cents
Thinkin' that an all-glass, kick-arse roof will not be fun with leathah....too hot or too cold, not to mention cracks after sunning from aft to stern (where did I hear boat tems?) with the roof(s). Other car has leathah and, unless heated and cooled, is just not fun for more that about 3 hours a year.