Download Illustator CS6

For CS6 back to CS4 you may download ONLY if you bought direct from Adobe via download
-Log-in to your account and click this link https://account.adobe.com/products
--click Order history on the left and then the link to your order that will show on the right
--and finally click the link "Downloads and serial numbers" that will appear
---as always, write the serial number in a text file AND copy that text file and the
---download file(s) to an external drive (or two) to be sure you have what you need
-if you did not buy direct from Adobe via download you must find your original install
-if you have an Education or volume license you MUST use your original media
--the link for an Education or volume license download is not available at all
-if you bought a disc and don't have a drive you will need to buy an external USB drive
-if you bought and lost a disc Adobe does not provide a replacement via download
-if you bought a download from a 3rd party vendor you will need to contact that vendor

Did you purchase downloads directly from Adobe?  If no, then you need the original disks / downloads you received at purchase + a valid serial number that exactly matches your media.   Failing that...

A subscription to Illustrator CC costs USD $240/year prepaid or $21/month for 12 months.

What's included:  software upgrades, 100GB of cloud storage, Adobe Portfolio, Adobe Fonts, and Adobe Spark. 

https://www.adobe.com/creativecloud/plans.html