where to find all textures online?

I'm looking for a site that gives textures that are a scale so I can edit them and put them in my WAD. I'm specifically looking for SPCDOOR3 but cannot find it online at all.

Is there maybe a way to copy and paste the texture from the game to edit it?

All of Doom's graphics and other resources are in the game wad. Get SLADE and use it to open Doom2.wad. The patch graphic used for SPCDOOR3 (under a different name, likely beginning with RW_) can be found in there and exported as a PNG.

Realm 667 has those textures.

check this out
http://forum.zdoom.org/viewtopic.php?f=37&t=24346