The Pell grant does help with online courses, but it depends on your school of choice, and it's not enough to pay for the whole college experience. Basically, it was about $600 that I received per year going through Colorado Tech. The problem you are facing, if I remember correctly is that Penn Foster offers a career certificate rather than a degree in Vet. technician program. When I spoke to a financial aid advisor about my options, I was told that Pell won't cover most certificate programs. Unless this has changed, you will have to find a way to pay for the courses yourself or find a school that offers at the very least an Associate's program.
There may be an alternative, though. Try talking to local vetrinarians and asking if they will allow you to work off your schooling in an internship/practicum type setting. This is what my friend did because it was something she really wanted, and in the state she resides in, you had to have a career certificate. Here, it would be more difficult unless you found a vet who wanted you to have training because you don't need to be certified in this state. Most people just enter the field as a vetrinary assistant because they have some working knowledge of animals, and learn as they go until they are ready to take a test to be certified as a vetrinary technician (two entirely different fields, though many think they are the same).
Another thing you could do would be to call the schools and ask them directly if they have a school code to use with FAFSA. If not, then the certificate probably isn't worth the money anyway. I know because I was taking a Children's Literature course through PCDI years ago, only to find out that it would just bring about a life experience, and wasn't a recognized certification.
If you're serious about going to school, there are some colleges out there that work with FAFSA, and offer you a degree. Being low income, you stand a good chance of getting further in your education by using FAFSA to your advantage and getting a degree rather than just a certificate. Certification is great, but doesn't usually pay much more than a high school diploma, from what I understand.