I have used sativas with very introspective trippy effects to interrupt and distract from memories and emotions that feed the flashback cycle... gotta be careful not to use one that's got heavy paranoia as it can cause even worse disassociation.
Heavy Indica can be useful for insomnia and to relax when your so tense that every thing seems to trigger... I get gnarly muscle spasms and use it to calm them down some.
Any herb seems to help with flashback dreams... doesn't stop them but I don't remember anything about any dreams so there's only all the sweat and a minute of panic when I wake.
I recommend a therapeutic approach as well as medication... it's really easy to start smoking constantly to avoid PTSD and eventually end up too stoned to function all the time or unable to get relief from marijuana any more. You don't want to abuse your medicine so much it stops working. I had enormous success with prolonged exposure therapy, it was unbelievably hard but after years of pain it was worth it. Took about 12 weeks of daily therapy that was HARD to do. But it worked. There ARE solutions to PTSD. Don't let it isolate you. That's the same tactic wolves use before the kill. Get some help to fight..
I get most of my seeds from ACE.