BioLegend and Invitrogen (ThermoFisher) have mAbs that react with Rat Foxp3 and are suitable for FC. You may want to explore our listings here: https://www.linscottsdirectory.com/antibodies?q=Foxp3
You can filter for Rat reactivity, and Flow Application; follow "more info" to supplier's data pages. Good luck.
I have been using Foxp3 antibody for mice for last 3 years. It is intracellular staining and you have to fix cells first and then you will add the antibody and later after the incubation go for FACS analysis. Here we use FACSARIA III.
remember one thing cells should be processed on ice .