If you have a pressure cooker whose sealing ring gasket finally broke, and you have one of those well-known brands, then you might have luck finding a replacement part at Pressure Cooker Outlet or Amazon.
If you have a lesser-known brand or even a generic pressure cooker, chances are those gaskets are typical in size. Measuring the inner diameter of the seal ring gasket in centimeters will be your guide. Some seal rings have the dimension embossed on them. I’ve noticed that many of the “universal” seal rings available for purchase have an inner diameter dimension of 20cm or 22cm.
Here are some options that claim to be universal which may offer several sizes (e.g. 20cm, 22cm and so on). An important thing to note is the material of your pressure cooker – some of the options linked below are not suitable for stainless steel pressure cooker as well as some other limitations, so read description carefully and do your research.