This is the only option we can recommend. Dog shelters have hundreds of dogs waiting to be rehomed, while breeders breed more and more puppies.
Of course, 99% of the dogs in dog shelters won’t be a Pug breed. But, there are lots of Pug rescue/adoption/re-homing charities and organisations, in almost all countries in the world. (Can’t find a local one on Google? Look on Facebook, they often have a Facebook page). These re-homing organisations (which are often just run by volunteers) and most shelters will require that you prove you know how to look after a Pug and have a suitable home, but at least you know you are giving a poor little dog a new chance at a nice and fun life.
If you want an older dog (and remember, there are plenty of advantages of having an adult dog as opposed to a puppy!), you can find them at dog shelters/pug rehoming organisations.
As Pugs are a pedigree breed, obviously there are many breeders. A quick Google of your town/area + “Pug Breeders” will probably turn up quite a few. You can also find them advertising on websites such as GumTree/Craigslist.
Most people would not recommend buying Pugs from pet shops. They don’t treat their animals well, they have no time for them, they just want to sell them on as quick as possible. Go down to your local dog shelter and spend some time with the dogs there before you think about just heading to a pet shop to pick up a Pug puppy…