Who can I contact regarding possible dog poisoning?

ASPCA has a poison control hotline that you can call. You would need to know what the dog ingested. You can't just call them with a random question and say, "Hey, I think my dog ..." Instead, you’ll want to say, "Hey, my dog just ate Devon-CON rat poison," or whatever it is, and they can tell you exactly what the active ingredient is and what you need to do. Again, your veterinarian will have to be part of that solution in 99.9% of the cases. But to at least give you some immediate information about how to handle things, and what you may or may not experience with that dog, what your dog will experience, calling the ASPCA hotline is probably the first step.

Call your vet if they're open, ask their advice, and go from there. Having the most information about what they could have gotten into, whether it's a certain poison meant for mice or rats or whether it's a cleaning agent, not just knowing it's the one that comes in the brown bottle, but also the name and the ingredients therein is very helpful.