
You should be fine buying a solid from a reputable breeder. You can find one through the Cocker Spaniel Club. It is true that there have been instances of irrationally aggressive cockers suffering a condition referred to as "Cocker Rage" and I think it is fair to say this has occured generally in the solids and not in the
particolours (I would want a whole one not a partial one :-P) but good breeders ensure they avoid breeding from lines where this may have occured.
Play safe, avoid puppy farmers, contact the breed club, book a puppy and be prepared to wait and you will not be sorry :-)