First you get a copy of it via Market or wait until it is available via AppsLib or Find it on a secondary source such as Slideme or...
Google and you will find multiple solutions.
The one I recommend is installing market on your Archos. Instructions are everywhere. Search for them and you will find them. Just go to Google
Search on: Archos install Market download You will find the answers
There is a Market Hack and see more Apps fix link in my sig.
Once you install Market, you should have no problem seeing Adobe Flash 10.1 in the Market.
I had problems with finding Flash too. Had to do the Market fix many times, then finally I was able to see Flash on Market. Many people are not able to find Flash.