Absent legal aid assistance or a pro bono attorney from a local panel (you can start with the local bar association) you will have to represent yourself. Fortunately, the support court, a subset of Family court, is streamlined and expects litigants to often be unrepresented, and is user friendly. Go on-line for advice on how to represent yourself in such matters, just so you know what you will need, file the petition, and if you are entitled to an increase, you should get it.
If you found this "helpful" or "best answer," please click it with my appreciation. My response is for educational purposes and does not constitute legal advice nor creates an attorney client relationship which requires all the details and a personal conference.
If, as you say, the father has filed a petition for custody, you, as the mother, are considered to be the respondent and are entitled to assigned counsel if you are indeed indigent and cannot afford an attorney. The Family Court Act, Section 262 mandates this. Here in Buffalo, representation is provided by the Erie County Bar Association's Assigned Counsel program.
The above statement is intended for informational purposes only and does not establish an attorney/client relationship.
I would contact your local dss, and ask for the support collection unit. You may apply for them to collect any child support payments. They may also aid you with any further modifications.
I understand your financial constraints but if an attorney can get you an extra $1000/mo, that is $12,000/yr. Over the next 10 years, that is $120,000. If he is a high net worth individual, he will likely have a highly paid attorney. It may be worth it to pay a good attorney to represent you. It is even more crucial if he is trying to get custody---lest you find yourself paying him child support.
You don't want a situation where your attorney can't provide you the vigorous representation that you deserve. He can always file a motion for attorney's fees.