A couple of recommendations:
spincare.co.uk - rounded or square inner sleeves. They can be a tight fit but records easily slide in and out.
I prefer the square sleeves because they are stiffened with a sheet of rice paper.
For £3.95 you can purchase their Record Sleeves Sample Pack which consists of 14 different inner and outer sleeves.
I used the inner paper sleeves until one crumpled and opened the seam, which is glued! After several hours I managed to remove the glue from the playing surface of an expensive record.
For the poly-lined, brown cardboard sleeves, swanflight produce an oversized record box (200 size only).
At £125.95 they aren't cheap but are built to flight case standards and available in different colours.
I have noticed that the white cardboard sleeves also vary in thickness.
I hope that the above is helpful.