I killed some hagbui runemasters at 9, 46k Vanern Swamp zone. There are two other places in the zone to find them (10k, 10k and 40k, 10k), but they had too many aggressive, roaming mobs nearby. Their respawn rate is low, and their drop rate is low (averaging one drop for every ten killed).
Level 45 turned-in 20 runemaster sealed pages for 156,000,000 experience each.