One of the easiest and best ways to resurrect characters is with the Amulet of Life. Generally it's found very early, and it can be used multiple times.
In 6, it's in one of the entrance chests and has 7 charges. You can find others elsewhere, but that one item could easily last you the whole game. (Personally I generally reload when a character dies, unless it's a crazy boss fight and I can manage to rez them before the fight is over.)
In 7, it's at the end of the beginner's dungeon, in the Ra-sep-re-tep's chest. It has 5 charges, but can be charged repeatedly (up to a max of 99) via the Recharge spell.
In 8, it's fairly random. You may find several in the various Arnika chests, or you may find none. Crock also has a chance to sell one. Really though, you're better off importing one. You can actually bring a whole bunch of them with you, but one is all you need. It has a random number of charges (up to 4) and can be recharged by selling it to a merchant and then buying it back.