Memantine is supposed to help with tolerance to adderall. Of course you have to get it prescribed first and also have to see if you can put up with any side effects you might get.
DXM is also said to help with tolerance to adderall, I personally don't feel too comfortable taking DXM on a daily basis while taking an SSRI due to the possibility of serotonin syndrome.
Chelated Magnesium is also supposed to help. I started taking this and actually find it helpful for sleep. Not sure if it has helped with tolerance at all, but great sleep aid.
You can get either one without a prescription from certain Indian online pharmacies, some of them open all day, and have reliable chemists. It's semi-legal in US and probably no one cares much about it in Canada but I could be wrong.
Both of these suggestions pretty much give you the best methods to prevent further tolerance/possibly reverse it and a means by which you can obtain the necessary materials. I'm sure it shouldn't be too hard to find an online source for anything you cannot get OTC, and being in Canada only makes it more likely you may find a domestic source online.
Personally I know a pretty decent site that actually offers a decent range of both z-benzos and benzodiazepines, ritalin, tramadol etc. Not all of them are indian. Aside from getting pharms to assist your cause, there are alternative
chemicals you could consider as well. Be careful about which you use, don't wanna get scammed or worse.
Aside from using tolerance prevention strategies you could always find a substitute for dexedrine that you can use basically so that you are able to take vacations/periods of abstinence from using dexamphetamine, helping add to your tolerance. The key here would be to incorporate the memantine and other tolerance reduction methods into use while using a substitute stimulant that will not exhibit a very strong cross tolerance/perhaps has a different mechanism of action since amp isi primarily a DA releaser that prevents reuptake via some monoamine effect. A typical dopamine/norepinepherine reuptake inhibitor perhaps at more mild doses since you wanna assure you dont incite cross tolerance.
I couldn't really say what you should use if you chose this, I am certainly no licensed doctor and I wouldn't tell you should do anything that may be in conflict with your ethics or the law. I know ephedra is often something I've heard of college students using to study with. Not sure if ephedra is legal. I looked up this
link for you, discussing modafinal and dex for purposes like yours and even asks about nmda antagonism for both. You may be able to get modafinal/nuvigil from your doc, if not the online pharmacies may carry adrafinal, the prodrug of modafinal that you dont need a script for
There are a few classes of chemicals that would work for this purpose, some belonging to phenethylamine classes like amphetamine, also there are cathinone family drugs that may suite you're needs for example Wellbutrin, piperidines like ritalin, "wakefullness enhancers" like modafinal or its legal prodrug adrafinal, pyrovalerones...the list goes on, but the classes are ones you can do some research on to see if there are any chemicals that meet your criteria.