[ubernostrum]

They still have those seats; they're branded as "Main Cabin Extra". They're not super expensive, but the price will vary depending on the perceived desirability of the specific seat you select (i.e., an extra-legroom middle seat probably doesn't cost as much as an extra-legroom aisle).

Most airlines also build free or discounted access to those seats into their frequent-flyer perks, though Delta has recently begun cutting back hard on that and treating it as a possible-but-not-guaranteed after-booking "upgrade" even for their top-tier customers.