Good Source for Metric Orings

Hate to divert from this tread, but I think my newbie style question is related, and may help others.

How are you all getting the sizes of these ORings for your guns? Are you ordering them after you’ve taken the gun apart and took calipers to each and every O ring? Are the manufacturers of our PCP’s listing sizes for the ORings in each and every gun? I guess bottom line, imagine yourself just receiving a brand new gun, and you want to put together your own seal kit. How are you all going about it?

Lastly, when choosing an O ring type, what type is the very best in an Oring? Viton? Buna?