Bestseller Sidebox - How to make it three categories deep only with relevant listings

I want to only show relevant best sellers about two categories deep.

I don't want bestsellers to appear on my home page--the reason is, it would be too irrelevant for most users on my site.

A few categories deep (2x to be exact), or deeper, I would always like bestsellers to appear, but I only want it to show up in those listings, or deeper listings.

I need to get best-sellers at a minimum to at least two categories deep.

Is there some kind of mod I can use or a reasonable way I can do this?

Here's an example/ illustration of what I'm trying to do:

e.g. if categories were in this structure:
level 1>level 2> level 3

and here's 2x example category trees:
software>games>action
computers>peripherals>keyboard

I would want the "bestsellers" box to appear on games/ peripherals/ action/ keyboard.. basically, I want the best sellers to show that level or deeper. Bestsellers wouldn't appear at all on the home page, or even pages such as "software" or "computers" (i.e. no bestsellers showing on 1st level category pages).

For example, let's say someone is on peripherals, I would want them to see the "bestsellers" and it would only display top-selling peripherals relevant to the category. Or another example; if they are on "games" they may see general top selling games, while if they were on "action" they would only see top-selling action games.

I've seen a lot of sites doing this, although I can't tell if they are using Zen Cart, or another ecommerce solution.

I am hoping someone has already had this issue & there's a mod out there, a simple admin option, or even plugin I can use to edit this.

I wonder if anyone else has had a similar issue