Hello Chris
Depending upon the size you require Cornwall Model Boats sell ready glazed portholes.
John
You can get glazed portholes in any number of sizes, also on ebay etc. Might not be economical if you need lots though and some of them come beautifully drilled for bolts ? Rivents which all need simulating after to fill the holes. Trouble with pre-glazed as I found myself recently is you tend to see a bit of brass where you can't paint where the glass is fitted. On the pre-glazed portholes I got last time the portlights seem to be glass or some form of really, really brittle plastic, on trying to "pop" them out for painting, they just shattered and crumbled.
Do you need them to be clear? What diameter do you need? (sorry, just seen 6mm). Have seen many builds over the years where the author just sliced polystyrene tube or brass tube into the required length and glued in place. Sand back flush inside, sand back to required profile outside. Cut the "half circle" rain deflector" above (don't know the word) from electrical solder wrapped round a dowell and cut in half. Sand it flat, or crush in pliers beforehand. When it's all sanded back flush inside you can use regular clear plastic sheet and glue inside.
....Just thoughts for options for you which might give some inspiration? I hate glazing, some glue always slips into view but at 6mm diameter I think you can avoid that somehow.
Rich