Protecting 3/16" wire rope going through poles?
A quick Google search using various terms eventually resulted in using "hole protector sleeves for 3/16 wire rope" leading to many links, one of which is Amazon:
VistaView Stainless Steel Protector Sleeves for 1/8", 5/32" or 3/16" Cable Railing

Being stainless steel, they are pricey and as Harper noted, unlikely to prevent water intrusion. One could use a silicone sealer when performing the installation to improve that aspect, but it's not going to be permanent or one-hundred percent effective. Temperature changes will pull humid air in over time.

You did not reference that the protection had to be done after the cabling is installed, and that's a good aspect, as I found zero links for after-install grommets or protection.