Take a look at Trex Arms as they are one of the better holster makers that support the FN 509 series. I have ran several holsters from them for both Glock's and Sig's and my first 509 I had gotten years ago but sold off due to not really having any aftermarket support at the time. I have since came back to the FN and the 509 series and that's all I own now. The make the Sidecar which is a combination AIWB and IWB that you can setup how you like. They also have the Ironside Hybride as well that is a combination of the OWB Ironside that can be setup as a IWB Ironside.
I am planning on picking up a Ironside Hybride for my FN 509 Mid size MRD as it wiill also fit my FN 509 Compact MRD as well and I can use it for a OWB or IWB holster if I don't want to use my Sidecar I have coming that is for both my 509 Mid size and Compact MRD's. I have used other brand name holster makers but I always seem to come back to Trex Arms for my serious EDC holsters as they have always worked well for me from snowy ass winters to hot ass summers in desert environments.
Here is a link to the Sidecar holster:
Trex Anrs SidecarHere is a link to the Ironside Hybride holster;
Ironside Hybride Holster