Where can I get this? I really want to play Halo Custom Edition because Combat Evolved is my favourite Halo and I think that this is a worthwhile investment.
You’ll have to look at websites like ebay as the game has not been officially sold in many years. You will also need to download the 1.10 Patch to allow you to use the Online Functions as the game no longer runs on the original GameSpy Servers. Personally the MCC version of Combat Evolved is now better than the PC port and is getting more and more so every update, not to mention mod support from 343 that is available in the Steam Tools Tab.
I see. Unfortunately, my laptop is a regular non-gaming one so it might not run MCC. Where can I get this 1.10 patch?
The Patch is available online, it looks like PCGamingWiki has it for download, as does the Internet Archive. It was originally distributed by Bungie, by that part of their website is gone now unfortunately.
Ok, thanks!
Wait do I have to buy it or do I need a product key (like Microsoft 365)? I’m confused
Maybe I can help,
For Custom Edition, look online for a link to download (Halo Maps I think has it) Custom Edition and Halo PC (Which requires purchasing a physical copy) need CD Keys in order to install. So you would need to get a key for Custom Edition in order to play it
Technically You can find a Key and use it online, but it is recommended to get Halo PC and use that code. 
However, MCC is a much better version overall, but I understand alot of older laptops not being able to run it. Hoped this helped