I've been playing l4d2 for for around 8 years now, but I wanna get into speed running. One of the problems is I'm not sure how to get the proper version of the game your supposed to run it on. Any other tips would be great too.

Man i was also having the same problem

So, great thing, it actually doesn't matter what version you run it on (in any case - you can run on any version) if you want the best optimal however - it depends on what you want to run - for the original 5 campaigns its best to go directly to 2.0.0.0 for example.

If you join the discord for the speedrun (linked on the left there <) theirs actually a channel with all the speedrun appropriate versions of the game - with information detailing why they are used and for what patch, additionally their is a tutorial playlist linked in their - which covers the tricks and guides + installation instructions for the old patches

Main Campaigns (Dark Carnival, Parish, Dead Center, Hard Rain, Swamp Fever) - version 2.0.0.0
Main Campaigns Co-op - version 2.0.4.5
All Campaigns Legacy (All Campaigns Except for The Last Stand) version 2.0.9.1
All Campaigns (self explanatory) - version 2.2.0.3

These are the categories for the main leaderboard, for more specifics like versions for ILs you can join the L4D Speedrunning discord (you should join anyways cause you'll download the older versions over there)