Look into invoice factoring services. You sell your invoices to them and get paid upfront. They get a % of your invoice and/or a small processing fee.
Ideally you want a non-recourse factoring agreement (the one where the factoring company takes most of the risk of non-payment), but it might be difficult to get. It helps if you can show that your previous clients paid on time. But then again the freelancing platforms handled that for you.
In any case, it's worth asking around. Have your lawyer review the factoring terms before you agree to anything.
Fortunately I only had this problem once, and for me, the way to resolve it was first, with the client. Nagging, explaining to them that if they do not pay, I will sell their invoice to a invoice collection agency, that, if they choose not to pay will affect their credit rating and reputation blah, blah.
The trick is to do this in a serious way, but not overly confrontational and to be clear that all it takes for them is to pay the invoice.
After 6 months of this dance, I said, enough is enough, and initiated the procedure of finding an agency, and then, lo and behold, they paid.
In retrospect I should not have waited 6 months, but your learn with experience. =)
Ideally you want a non-recourse factoring agreement (the one where the factoring company takes most of the risk of non-payment), but it might be difficult to get. It helps if you can show that your previous clients paid on time. But then again the freelancing platforms handled that for you.
In any case, it's worth asking around. Have your lawyer review the factoring terms before you agree to anything.