You can buy old, sealed retail boxes on eBay. But verify the activation key works before paying. Expect to pay $300-$500 for a genuine, unopened Master Collection.
If you need a permanent solution, explore Affinity Suite (Photo, Designer, Publisher) for a one-time payment alternative. If you need the industry standard, subscribe to Creative Cloud. But leave the ghost of CS6 in the past, where it belongs. adobe master collection cs6