I've now released all the supplements as separate game purchases. You can see them all at https://itch.io/profile/half-apress.
This also allows you to buy a particular supplement out of the recommended order of acquiring them. Do so at your own discretion -- however if you're just looking for a source book for a specific purpose, such as generating new star systems, you can get the supplement that does that (A Lot of Zeroes) without having to buy everything else.