Where can I find lots of petty creatures to charge my soul gems?

Solution 1:

The Cronvangr Cave in Eastmarch is full of weak spiders, I usually hunt there.

Map, courtesy of the Elder Scrolls Wikia:

Solution 2:

Look up info on the questline to get the Black Star, which is a reusable soul gem that will take a humanoid soul (which seem to always be grand). Then any old bandit can be sucked up into that thing.

Also, draugrs and skeletons and the like are White souls, so you can just go in a dungeon with a bunch of those guys (with a weapon enchanted with soul trap) and suck them all up into gems.

Solution 3:

The easiest way I've found to get petty souls is simply to kill random creatures like foxes, deer and goats. Nearly anything that moves can yield a soul, and you'll find these creatures in tremendous supply throughout the world just in the regular process of moving from place to place.

Solution 4:

The best way I found to fill up petty soul gems (and fast) is starting from whiterun and following the river down killing all the mud crabs I find. They are easy to kill and will attack you if you approach them so it a good excuse to disintegrate them (as i dont like killing creatures for no reason) xD.

Solution 5:

I've had a great deal of success with the wanton slaughter of chickens and other livestock. Be advised however, some companions won't stand for it. I tried it with Lydia following me, and she left my service, and tried to kill me.

And of course, if there are any other witnesses, you could end up with a bounty and some angry townsfolk.