Auto parts stores
or even welding shops
I get filled at local auto parts store. Big name local auto parts stores most likly have a nitorus fill station in the back.
When i bought my kit I also got the LED illumitated purge kit. Shoots a 15' jet of nitrous that gets illuminted by a very powerfull LED. wish i have a pic of it at night. Looks too cool. And plumbed it in when i installed it. Mine is a blue jet coming out of the wiper cowl area. Triggerd by a push button on the dash