How to find the money for an Open Access publication fee, if you're a postdoc and you don't have any funds?
I see several routes to proceed, in the order in which I would suggest that you attempt them:


Check if your former or current institution has an organizational affiliation with the journal that will allow you to publish there for free (i.e., the OA version of a subscription).
Ask your current supervisor.  They might be willing to pay despite not being an author, provided that you list you current affiliation as well as your former (which is generally reasonable to do).
Many universities now have an open access "slush fund" to help deal with situations like these.  Check with the department leadership and the libraries at both your current and former departments and see if there is such a fund.
Contact the journal and explain the situation: they may be willing to waive the fee.
Publish the paper in a "traditional" journal that has no fee.