When it comes to websites I'd suggest a lot of reading on Australian involvement in Vietnam. For clothing etc you'll be looking at eBay, collector's fairs and reenactment sources. For social media groups I'd suggest a more general approach and look for Vietnamese reenactment groups and I suspect you'll also find a few airsoft ones too with a bit of digging.
Best of look sorting a load out etc, unfortunately I don't think there is an airsoft version of the Owens SMG which is a bit of a shame but if you talk to Skunk Airsoft Works then you maybe able to come up with a plan for a custom built one between the two of you.
Pursuing an impression load out is a very personal venture and while there are various dedicated forums and social media groups there is an expectation for you to have carried out some of the preliminary work yourself, if not slightly more indepth research.