AEG'S STARGATE & FARSCAPE BOOKS
When I originally ran Stargate, I was still a college student and had been for several years, so I had no choice but to be frugal. The books were also fairly new, so finding them used or at a good discount was a quest in and of itself. But many years have passed since then.
AEG produced a core book for Farscape in 2002 and a core book for Stargate in 2003. Four supplements followed for the Stargate line, but only one book was ever made for Farscape. Eventually, they lost the Stargate license and the last book they planned to do was never made. The Farscape book, Stargate core book, and First Steps supplement are all currently available in PDF through DrivethruRPG.
In 2020, Wyvern Gaming ran a successful Kickstarter to create a new, officially licensed Stargate book via the 5e Open Gaming License. This book has the advantage of being made well after the show's completion (along with its spin-offs). There's a wealth of material to choose from and the creators won't risk going against what was established on the series. It's likely to be the gateway for people to play Stargate after its release, since 5e is a very popular system.
But I don't know if I'll ever run the new system, and I enjoyed AEG's version. There was never a lot of information about Stargate games online, which is part of why I started this little site in the first place. What I came up with while running could still be of use to people, too. Things you find here could inspire some ideas that, with some tweaks, will work just fine in 5e. And after Wyvern Gaming publishes the new core book, I've only seen that they plan to publish materials related to their living RPG series (Stargate: PHoenix) on their forums. If there are plans for supplement books, I have yet to hear of them. So you might find value in tracking down older materials.
The Products
The Stargate SG-1 core rulebook book is a tremendous resource. It has information you can use in Stargate games and other modern d20 games, as well. It is about 500 pages in length, in full color, with tons of content and some pictures and diagrams for reference. It also has a decent index for a book of such a length. What's more, you really don't need much else to play the game. You don't even have to have the Player's Handbook with you, if you instead use the SRD available for free through Wizards of the Coast. (See the links section for a link to a hyper-text version of the SRD.)
The four supplements were all printed on on the same nice paper, fully in color. They all present information that is not offered in the main book, so it's not like you're buying redundant products. Most of these books present game mechanics for things that have been shown on the television show, as well as characters. All of the books have new information, however, and one of them (I forget now which) features extended information on vehicles, which is missing from the main book.
FarScape Roleplaying Game (2002; available on Drivethru)
Stargate SG-1 Roleplaying Core Book (2003; available on Drivethru)
Stargate SG-1 Fantastic Frontiers Season One (2003)
Living Gods: Stargate System Lords (2004)
First Steps: The Stargate Unexplored Worlds Sourcebook (2004; available on Drivethru)
Friends and Foes: Stargate Season Two (2004)
A Suggestion
You will need the core book to play, but the extra sourcebooks are just that - extra. They are good for more definition and flavor, but you can play quite well without them. Watching the show again can reveal a wealth of uncovered ground and plot hooks, particularly if you expand your viewing to include Atlantis and Universe, neither of which were hinted at in the RPG. If you want to run Stargate in the later years, you'll also want to hit up guides to the show's later seasons, since so much happened that the game did not get to cover. In addition, if you run other space-faring games, those books will likely have things that you can throw into Stargate, or you could take Stargate's millieu and run it in a different system altogether.
If you run AEG's Stargate, it helps to have at least two copies of the core book. One reason for this is that the core rulebook has an amazing amount of gear, and players can choose new gear once per mission. Having more than one book for players to use makes this stage faster. Another concern is that there are a great deal of different combat rules, feats, and skill uses that players may need to look up for certain situations. Whether you have PDFs or printed copies doesn't seem like it will matter much.