I'm pretty sure we had a whole thread filled with these a while back. Not sure if they'd help with finding more, but it would at least provide some inspo Looks up Yuca's alley as well! That Sunspel looks really nice...
Cheers BB - it's not know what to search for on Etsy/ebay that is the frustrating thing... Leisure shirt was my first instinct...?
"Loop collar" might be a keyword to try...
https://www.ebay.com/itm/VINTAGE-50s-60s-PENNEYS-TOWNCRAFT-ATOMIC-ss-shirt-LOOP-COLLAR-rockabilly-mens-m/264237623613?hash=item3d85ca293d:g:MKoAAOSwpCFciRkW:sc:USPSFirstClass!10034!US!-1
^ Thanks TC, nailed it. On the tip of my tongue.