Usually the prior edition is just fine. this is a very good point to make. Most of the law they teach you as 1Ls is pretty well-established and isn't changing year-to-year. Combined with your professor's lectures, you're generally fine.
be sure not to get your supps very old, though. You'd want to know if the Federal Tort Claims act had been amended, for example, and a supp from 1990 may not be good for this purpose.
One thought: look at the year your casebook was last published and you can buy supps from that year forward.