After years the o-rings on my Fireti Omega wallet broke down and I thought it was just a matter of going to the local hardware store and replace them. But you really need a specific size to get this working.
The diameter depends a bit on your personal preference how many cards you like to put it, but the most important thing is that you use the correct thickness which is 2.0 mm. By default the inner diameter of the o-ring is 10.0 mm. I prefer 12.0 mm because it makes the wallet more easy to use.
Please also take into account that you buy some sturdy o-rings that can handle everyday usage, a good example is the EPDM shore rubber with a 70 hardness. These rings will not brake that easily.
The correct size and type o-rings
O-RING-12×2 EPDM 70 SHORE (12.0 mm diameter) or
O-RING-10×2 EPDM 70 SHORE (10.0 mm diameter)
The are available at specialist shops or on the internet, as a guide I bought them at Technirub, please follow this link.