The coolant crossover pipe o-ring as used on all EJ engines is an NBR 33.7mm x 3.5mm JIS P34 spec. The durometer spec is either 70 or 90. Pricing on this o-ring is around $5, but could be purchased in bulk from specialty suppliers for under $0.25 each.
Subaru OEM part number: 806933080
Felpro aftermarket replacement: 36024
Bulk options: