They have been sporadically on sale at Amazon; I picked my set up from there at the beginning of March. I have never seen them for sale in a brick & mortar store, although I stopped looking after I got mine online. But right now I think Hasbro’s shop & Amazon are your best bet.
Here’s the Amazon US link to watch, if it helps.
Hasbro Gaming Dmx Dropmix Discover Pack Series 4 Electronic Game https://www.amazon.com/dp/B072XWVYND/ref=cm_sw_r_cp_api_e0kZAbHRHVQRB
Amazon.com.au Lucky: https://www.amazon.com.au/DropMix-Playlist-Pack-Country-Lucky/dp/B06XKQCHRF
Chiller: https://www.amazon.com.au/gp/aw/d/B07BL5T6L7
Amazon.com (USA) for the rest with a freight forwarder Too much $$ haha. I tried to buy from Target.com to a freight forward but they kept canceling my order. So I had to pay a bit extra.
The Toys R Us exclusive is obviously higher prices by scalpers.
The Discover packs are retail price https://www.amazon.com/gp/aw/d/B072XWVYND https://www.amazon.com/gp/aw/d/B072XYCV6N
Edit: Amazon.com.au now delivers the Discover Full set 3 & 4 to Australia. Free shipping with Prime. Cheaper than buying in USD and freight forward. Search "dropmix discover" on amazon AU.
You will still need to buy the Target and ToysRUs exclusive from EBay or Amazon USA as the Amazon AU sellers are really high prices.
I was just able to order :) Hasbro Gaming DropMix Discover Pack Complete Series 4 30-Card Bundle https://www.amazon.com/dp/B072XWVYND/ref=cm_sw_r_cp_api_glt_fabc_Y3R9GDRDV12RM4SJ58ZG
Both Series 3 & Series 4 are back in stock as of this morning; after I just bought one of each, there are 18 left of Series 3 and 10 left of Series 4.
Series 3 - https://smile.amazon.com/gp/product/B072XYCV6N
Series 4 - https://smile.amazon.com/gp/product/B072XWVYND
(edited to add links)
https://www.reddit.com/r/dropmix/comments/8brklv/whats_with_hasbros_page/dxas2qe/
We're working on getting more back in stock. That being said, be sure to check around other online and local retails for them!
With that said, I recommend ordering the combined Series 1 + 2 pack from walmart.com via this link. The description is wrong - it says 5 cards but it is the complete set. The shipping time is also misleading - I got mine the very next day.
https://www.walmart.com/ip/Hasbro-HSBC3411-Dropmix-the-Discover-Pack-Display/672215481
As for series 3 and 4, they have only ever been available via Amazon and Hasbro Toy Shop. Both are currently out on Amazon, and Hasbro Toy Shop only has 3, so unless you want to brave the resellers on eBay, you can't get 4 at the moment.
In case you want to watch them - here's the links to the out of stock Amazon products:
Series 3: https://www.amazon.com/Hasbro-Gaming-Dropmix-Discover-Electronic/dp/B072XYCV6N
Series 4: https://www.amazon.com/Hasbro-Gaming-Dropmix-Discover-Electronic/dp/B072XWVYND
They are getting more out, slowly but surely. Within the past week or so, the first playlist cases with Lucky included finally started showing up at locations other than Hasbro Toy Shop, Amazon, and Target.
Good luck in your search for cards!
The Amazon link, no thanks to OP: https://www.amazon.com/Hasbro-Gaming-Dropmix-Discover-Electronic/dp/B072XWVYND
Hasbro Gaming Dmx Dropmix Discover Pack Series 4 Electronic Game https://www.amazon.com/dp/B072XWVYND/ref=cm_sw_r_cp_api_kmMMAbMJ33HQ5