You can dodge some of the worry...
The suggestion was made in an earlier post that the problem may lay with the official cartridges and juice. If that's the case, then the spiking is going to be happening at either the ingredients manufacturers or the cartridge and juice manufacturers.
Someone mixing juice in their garage isn't what I'd call 'official'
The big suppliers of the ingredients, the actual stage one manufacturers, aren't going to be selling retail. They'll be supplying wholesalers, who supply resellers, who bottle up stuff themselves - and then they may retail, or they may go through another level of resellers / labellers.
Personally, I reckon it's just as 'safe' to be buying reputable premade juice as it is to be buying reputable ingredients. The premade juices and cartridges are simply swapping that last reseller stage or two.
If anyone is saying that "Dave's joint juice, get high in 3 tokes" is an official source, then I'm obviously out of the loop...