First, it depends what you mean by post-bacc(which is really just another word for post graduate)
A) You have completed your pre reqs, but you need to raise your gpa.
B) you have not completed your pre reqs,
Many people in situation A apply into a Master's program (usually in bio). The classes are upper level bio and by doing well, they raise they're overall gpa and show an upward trend.
They same could also be done by taking upper level bio class at a 4y college as a non degree student. this obviously would not yield a degree though.
people in situation B will have to complete the pre reqs as a post grad. there are two options here as well:
Formal post bac- these programs need to be applied into and can be very competitive. they have planned schedules and advisors that assist the students with their needs.
OR
Informal post bac (what I did) - I applied to a CUNY school as a non degree student and simply signed up for the classes i needed to take. i didnt have an advisor, but i knew what i needed to do and planned accordingly.
hope this helped