BORROW THIS IDEA: One of my brides shared a link she found for vintage postcard Save the Dates so thought I would share it with you all. It's $99 for 100 4"x6" cards that you can personalize. Pretty good deal.
Otherwise if you have your own graphic file, I sometimes use Overnight Prints which is only $12.95 for 100 4"x6" cards.
HELPFUL HINT: Remember only send save the dates to people you really want to save the date! Don't send one to someone who is on our B list if you're not sure you're inviting them to the wedding for sure. It totally sucks when you have to explain that you have too many people that are coming now and have to retract the initial invitation you extended to them!