I do not advise the procurement of street amphetamines for the treatment of subjective ADHD. You would need a scale to reliably measure out doses; also, you can never be certain of the potency of what you're getting.
Methamphetamine is the common analog of methamphetamine used on the street; ostensibly, you can compare the follow differences between an oral dose of 25mg. of racemic amphetamine salts (Adderall) and 25mg. of methamphetamine:
1. Methamphetamine has a lot more SERT activity; this could cause you to lose focus as it promotes more well-being/euphoria and counters the DA/NE release.
2. Methamphetamine will have a longer duration and take longer for onset.
3. Methamphetamine will feel more potent, 1.5x as much; however, methamphetamine will feel a lot smoother and honestly a lot more enjoyable than Adderall.
I would advise, as a pure study aid, something that pushes only DA/NE release with perhaps adrenal effects-- this way you're "forced" to study rather than experience pleasure. Also, from a harm reduction standpoint, street amphetamines will be more toxic than their pharmaceutical counterparts.
Tread wisely.