I know many model railroaders also like to shop at art supply stores for items like paints, airbrushes/airbrush parts and scenery materials. I recently bought some airbrush parts for my Iwata NEO (a larger .50mm needle and appropriate-sized nozzle and nozzle cap) from an online art supply store in Oregon called
The Merri Artist.
Not only are their prices better than other online art supply stores, but they had a website that showed exactly which parts were needed to upgrade one's Iwata NEO to have a larger needle size (ideal for working with acrylic paints), with the appropriate "Add To Cart" buttons. And furthermore, their shipping prices were perfectly reasonable for the size and weight of my purchase. None of that expensive flat-rate nonsense. Another perk for me personally, since they're in Oregon, they shipped quickly to here in CA. My order arrived a day earlier than expected!
Highly recommended! They also sell relevant modeling/weathering/scenery items like cutting mats, India Ink, gesso, pastels, oil paints, brushes, matte medium and epoxies/resins.
themerriartist.com