I think a lot of vendors carry generic Desoxyn at very affordable prices.
Desoxyn is pharmaceutical methamphetamine.
Vyvanse is lisdexamphetamine, a slow release version of dexamphetamine.
Don't be mislead by the name, they're quite different. And methamphetamine is very rarely prescribed, only when everything else fails. While Vyvanse is usually first line treatment.
Methamphetamine seems to be very effective with fewer side effects, but there's a lot of stigma and legal issues with it being one of the most controlled substances. So even if cheap it's not going to be easy or convenient to get it prescribed.