You should be able to get them from most banks still. They most definitely do exist, but the majority that are in circulation aren't worth anything more than their face value (1976 and 1995 series). Anything older MAY be worth more than face value, especially if it has errors or is a replacement bill, but it'd probably have to be in mint condition, and you'd have to find the right person to buy it.
I wouldn't bother with hoarding them though. With the amount of them still in circulation today, they probably won't be worth more than their face value anytime soon (i.e. in our lifetimes). Just spend them, I used to leave them as tips.
