I need fixer and psycho

Go to the great khans or that dealer in freeside

The guy in the trailer in the Khan area comes to mind. Dixon in Freeside perhaps. I'm sure there are others but I don't use chems too much so...

Some of the doctors frequently sell fixer (I know the one in Novac does).

Still going to be somewhat random & they rarely sell more than a few at a time so you have to keep going back over & over to get a good supply.

I'm doing the High Times quest. Dixon in Freeside has none.

It's random...sometimes they'll have some, sometimes they won't. Every three days inventory changes so you have to keep checking.

go to the bartenders at gomorrah and loot some fiends

Killing/looting fiends isn't a bad idea. Sometimes viper bandits drop chems too...but that's so random probably not worth the effort.

double posting

I forgot...the doc in Novac alters inventory every 24 hours, not 3 days. You can just Wait in front of her until you have the Fixer you need. Not sure if other docs are 24 hrs.

Are you trying to collect 10 fixers for the guy? That's a horrible task, you should have picked the Speech option instead.

there's a merchant at 188 Trading Post who has 10 fixers