Why does my puppy constantly have seizures?

He is a five month old maltipom. My sister bought him from a breeder far away and he was 'shipped' here. When we first got him, he had a seizure so we took him to the vet and we were told that he had pneumonia. After that, we took him to the vet for a couple weeks to get him treated and he was fine. But after he recovered from phnemonia he still has seziures like once ever two weeks. Why is this? He will start foaming in the mouth, spazz out and sometimes he will pee/poop without knowing. We can't afford to take him to the vet right now. We really need to go, I know. We are trying to see if he gets any better or if it gradually stops but for now, I want to know what is happening. Do any of you experience seizures with your puppies? I need help. Please help. Thank you.