ask friends / family, often you are connected to a lawyer closer than you thought.

If you still don't have a free/cheap resource then start CCing your lawyer on emails. Eg. 'I am writing to provide notice that you are now 60 days overdue on invoice X (attached). As you have refused to pay the invoice under the terms of our contract (attached). I am copying my lawyer on this email to bring this matter to their attention.

Edit: and also make sure your contract spells out what happens for nonpayment.