I have bought several sizes and found that 6x1mm and 7x1mm are correct sizes. You can buy them at AliExpress:
https://www.aliexpress.com/item/100Pcs-Black-O-Type-Sealing-Rubber-Ring-Gaskets-4-5-5-5-5-6-6-5/32725251085.html?spm=a2g0s.9042311.0.0.ZkdS1C
100 pieces for less than a dollar, shipping...
I charge 3 10180 cells at once at 200mA, using little magnets at both ends to place the cells in series and some more magnets to make the battery fit in a slot. Each cell is then effectively charged with 65mA.
One downside of this method is, the cells have to have about the same voltage to start...