The area around [ latitude-59 longitude-44 ] is a very good place to find them. They can be found in swamp areas elsewhere, but where I mentioned above seemed to have the most spawn.
Be sure to kill anything dangerous or even vaguely scary that's around the area, or the Sinomacrops will freak out and fly away.....and trust me once they flee they're as good as gone, given how small they are.
I know I’m a bit late but if you want a sinomacrops early I suggest to spawn at boskar beach there’s a swamp there which lots of dung Beatles, dragonfly’s, and snails they also spawn pretty commonly there too just be careful of snakes kapros and barys
Having trouble finding them along the beaches these days ? No problem , go to bottom right of lost island map 80 90 80 90 that grid square has all the ones I tamed .....full of predators wear ghile armor when feeding