While the engine and game code of Quake were open-sourced and released for free, the game data and music were not. Thus, if you want to play the full version of Quake, you will have to buy it. If you just want to try a demo, there is a so-called shareware which is free and includes the first (of four) episode of the game.
You could find an option to download Quake but since the music is not included in those releases it is not recommended.
Right, you are still reading this so you want the real deal. Excellent! To buy a "physical" copy of Quake you will have to look for second hand options. eBay obviously comes to mind, so does Amazon and maybe you are know some smaller independent local stores too. Craigslist would probably be a good choice if you are in the USA.
Do not pay more than ~10 € respectively ~15 USD for the plain disc. And not more than twice that for a box. Even if the condition is great and the seller tells you it is so super rare (it is not). Just be patient and you will be able to get it cheaply.
Hopefully at some point in the future this site will have a detailed list of all Quake releases there were but for now just be aware that as alternatives to the original boxed or single disc or shareware release you could buy Ultimate Quake (which is a bundle of Quake 1-3) or Quake: The Offering (Quake with the two official missionpacks). They all include the full game and the soundtrack.
Use this as search terms and limit the results to PC videogames:
quake -"quake 2" -"quake 3" -"quake 4" -"quake ii" -"quake iii" -"quake iv" -territory -"quake wars"
This link should get you there. Don't forget to check your local eBay first!
Amazon does not let you filter as nicely but since items are bundled per release you can still find what you need.
All "quake" in the PC videogames section. Don't forget to check your local Amazon first!
At the time of writing the following releases were available, but please go through the list manually. Sometimes sellers on Amazon enter weird things (see the two Ultimate Quake entries below for example).
Also look at other categories, for example there is an entry for Quake in the Books section…
Always make sure to read thoroughly what really is offered.
This is an awfully bad offer but you can buy a digital download from id Software themselves as well. Costs 20$ and surely does not include the soundtrack. http://www.idsoftware.com/games/quake/quake/
Check local gaming forums and communities. Ask in our forums.
Quake is banned in Germany so eBay and Amazon do not list it. You can get lucky at eBay though, if you see one offered just mail the seller, offer 10€ and you will probably get it after eBay deleted it.
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Are you currently following the Quake setup guide? You own Quake? Proceed to the next step: Quake Installation