By 'working or cocker spaniel' do you mean a working spaniel, either springer or cocker, or...?
ChampDogs is a good place to start as at least one parent of the litter will have been health tested. I've known some very good litters be put on ChampDogs.
Pets4Homes isn't where you will find the A* breeders, but you can find some decently bred litters there: you will just have to do more due diligence to weed out the puppy farms and the people who haven't much of clue what they are doing.
If you specifically want a dog from working lines (and I assume you know what you're getting into, I love working line gundogs, can't imagine owning anything else, but God, are they hard work as puppies...) you could maybe approach some gundog trainers in your area and see if they know of any litters due or planned. Responsible breeders should not be hiking their prices to the insane heights seen this summer, but have a clear idea about what you want to pay.
And please, please be aware that working spaniels are being stolen in ridiculous numbers at the moment, so check very carefully the origin of the dam of any litter you look at. That's one good reason to get recommendations from a gundog trainer who will know his or her client base and shooting buddies, and their dogs.