TF2 maps are stored in this folder: C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2\tf\maps. However, they are stored in a format that cannot be edit in hammer called a BSP. To turn the map into a VMF, the file type that you can edit in hammer, you have to decompile the BSP. This can be done by using the tools listed here: https://developer.valvesoftware.com/wiki/Decompiling_Maps
As mentioned by Hyperion, https://tf2maps.net/threads/valve-maps-decompiled.5952/ is a great way to get all of the VMFs from the official maps.
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