If you need a post mortem on a dog it can be possible. The ideal situation is for it to be done in less than 24 hours from death. Less ideal is for the body to be frozen as soon as possible.

Most PM places will not accept a body directly and your vet must 'refer' the animal. It will cost circa £500 or more. There are not many high quality certified places and they are usually either connected to a university vet school or are part of the APHA lab network.

Discuss with your vet the presumed history of death in detail. Pathology does not work in isolation but will look for clues based on what you're looking for. They cannot do 'tox screens' for every possible substance not knowing what to look for. It is more likely that the PM place will look more for supporting evidence like liver damage etc than direct toxins.

Tl:dr - freeze the body, speak to your vet to get referred on.