Advantage is OTC. But we're not allowed to give medical advice here, a) because most of the members are students, and b) those of us that are licensed can't violate client-patient relationship ethics and give pharm advice about an animal we haven't seen.
If finances are that tight, try consulting a veterinary pharm book (e.g. Plumb's - although the dosages and route might be an issue with the packaged product), or calling the company directly. But be careful. A cat is not a dog is not a rabbit, and doing things like trying to split doses on ANY medication, if you need to depending on the rabbit's weight, can be very tough in pre-dosed products and you risk over or under-medicating.