Nikola
I use slugs a lot, but importantly I only use ones from a safe source. Slugs coming out of a drain would carry traces of any chemicals that had passed through there. The same goes with wild caught ones, mine all come from a friends garden who does not use any form of chemical in their garden. You also need to examine the slugs carefully to see if they have the blister like swellings on the saddle. These are infected with worms and should not be used. I would also not use any showing signs that one had been there and ruptured.
If you go out on a wet/rainy night and collect any slugs you find. You can store them in a suitable plastic container with lettuce leaves or dandelions etc. I have kept wild caught slugs up to 2 weeks this way. You can also dust the food you give them with light concentrations of calcium and vitamins if you wish. You do need to do it sparingly though as the slugs will leave the food if too much dust is about.
I hope this is helpful.
Jeff