I wish there was a vendor somewhere specific or in front of each old school raid where you could buy LFR armor with either money or tokens you obtain doing old raids either solo or with a group.
Which raid?
There is dude in front of dragon soul for LFR transmog
There is panda in mogushan palace you can to talk to run pandaland raids LFR transmog
Talk to some npc in your garrison for wod LRF thingies.
There is mage in front of voilet hold in legion dalaran for legion LFR transmog
You can queue for LFR raids of old expansions for most older raids as far as I know. The npc to talk to will vary depending on the expansion.