You might look into this:
Free Student Software Downloads | Autodesk Education Community
It is also possible to get older editions of ACAD by way of Ebay and/or used software stores. The older stuff is pretty inexpensive and for messing with it or formal study, even a 5-10 year old or older edition will be fine.
ACAD has largely gone to a subscription model and regular users tend to use the latest editions and toss earlier ones.