With all the Space Marine chapters out there, lots of players can play "the same army" but have wildly different backstories for their forces. Thankfully, a Space Marine model is pretty modular. There's a lot of bits you can swap out to easily distinguish your chapter from your friend's. Same goes for their vehicles, helping you differentiate different versions of those as well. Forge World is here to help with all of that, particularly if you want some Thousand Sons pieces.
You've got several sets of Thousand Sons shoulder pads you can pick from, as well as a set of torsos and a set of heads. If you want to go full-on and get entire figures, you can pick up the set teamed up with the Betrayal at Calth game set. If Thousand Sons isn't your thing and you'd prefer Red Scorpions, there's a set for that, too. Finally, you can get yourself a Damocles Command Rhino upgrade kit, to make sure all your troopers know what they need to be doing out there.