Actually, it is something you can buy and just start using, if you like. Rapa is available at ADC.
Interview with a doctor who's using (and prescribing) it now:
Interview
The doctor's website
Side effects (canker sores in mouth, etc.) appear to be linked to every-day dosing (for immune suppression, as is used to prevent organ rejection.) This may be because rapa has a long half-life in the body (62 ±16 hours), so daily dosing causes major spikes in blood levels as the peaks begin to stack.
Rapa anti-aging protocols call for dosing at weekly or longer intervals, providing an adequate wash-out period. (Interestingly, apparently rapa boosts the immune system when given at this cadence.)