According to the books in the dragon guild, the following food items are recommended for helping a dragon/fire hatching player to raise their resistances.
Delicious meals for hatchlings:
zombie's corpse
ogre's parts
para elemental residues
ghost's ectoplasm
scorpion's stinger
Delicious meals for wyrms:
beholder's tongue
demon's head
elemental residues
wyvern's steak
Delicious meals for wyverns:
skull's tongue (mjam!)
wyvern/dragon steaks
Does anyone have any corrections or updates that should be added to these books?
As you get higher the quantity and availability of food approaches 1 on purpose. This is to make it harder and to slow it down - remember that once your dragon has 95's in all his resistances it's a very powerful ability.
Also as you get higher it tends to be foods that work for any of your resistances as there are less foods available. It's been a long time since I was looking for food around that level but I believe you should poke around in ancient pupland. You will find one food source. If memory serves me correctly there are 16 of these monsters guarding 3 of another type (which those 3 do not taste good) in Ancient Pupland.