As far as I know, there are no aftermarket battery packs available. Cheapest option currently is to buy a wrecked car with a good battery and swap it yourself. I doubt Nissan will sell you a pack but who knows? Eventually I assume someone will come out with aftermarket battery kits but I haven't seen any. Replacement at the dealer is currently around $6k I believe.
There are several guys here at work who have 50k+ miles on their Leaf's with all 12 bars remaining. Not all of the battery packs die an early death and if you get one with a 8 year warranty, it will get replaced for 'free' if it does go bad. Usually they get replaced at around 60-65% of full capacity so if you only need 40 miles, you would be covered in all but the worst weather.
I agree the used prices are nuts in Boulder these days. I bought a new one last year since it was cheaper than the used ones. First time I've ever bought a new car in my life. And I've been driving for 40+ years