There are thousands of express passes sold every day. there is a limit of course, but on busy days they usually last at the parks until between 11am and 1pm before they are gone. moderate to slow days they are usually available pretty much all day. as for pre-purchasing them online, the amount set aside for online purchase are extremely limited, which is why they sell out online so fast, but sold out online does not mean sold out completely. so long as you arrive at the parks by 11am you should most likely be able to get express passes. just keep in mind the pricing can vary day by day. once you are at either park you can also pre-purchase for another day, so if you know another day you will be there you can buy your express for that day also, eliminating your need to arrive early that second day.
