For about 25 dollars more you could get annual passes(instead of a flex pass) and try to squeeze in an extra vacation before they expire. You will need to check on blackout days etc. plus the rules and regulations for each. This possible solution is especially nice when you already have one of the above.
yes you can buy them at the gate at each of the parks although I know the Orlando Visitor Centre on International Drive (next to Pirates cove golf) has them a few dollars cheaper than at the gate