There are 2 sets of o-ring, looks like you are looking for 97:
O-Ring for Filter adapter:
ID OD Width
GM part number 12559583 smallest o ring is standard o-ring 115
115 11/16 7/8 3/32 0.674 ± 0.009 0.103 ± 0.003
GM part number 12559584 medium o ring is standard o-ring 215
215 1 1/16 1 5/16 1/8 1.046 ± 0.010 0.139 ± 0.004
GM part number 12559095 largest o ring is standard o-ring 235
235 3 1/8 3 3/8 1/8 3.109 ± 0.024 0.139 ± 0.004
This is from my note, found it on cyberspace somewhere.
Please verify before you get it but as bison said you can get it from local hydraulic shop.
For online supplier check with Leroy Diesel.