I've subscribed to several magazines, and can only access issues related during my subscription period. I also must archive issues to bring them back if I want them off my device. I'd love back issues of some of the old magazines.
Don't believe that there is an option to purchase older issues of magazines. But it's a good idea though. Maybe you should put it up on the Ideas forum.
Since you're new to this, two other pointers:
1) If you want to save past issues of magazines, get in the habit of archiving and unarchiving issues right away so you always have access to the old ones. If you don't have them archived, I think it starts deleting older issues once you get to 10 (if someone else could clarify the count for the OP, it would help.) I think if you have archived them though, you always have access. (I know I read something like this a year or so ago, don't know what current status is.
2) Some of the magazines follow a "publishing schedule", not a calendar schedule. For example, the February's issue might be released on January 15th (or whatever earlier date.) Point is, you don't always have until the end of the month if you are buying one-off issues. (Doesn't matter if you have a subscription.)
Hope that helps.