There are probably many resources but I think some of the Optometry schools have listings on their websites. UCB for example has quite a list that i feel is more organized than most of the other schools. It is also updated quite frequently from what i have seen:
1. State Optometric Associations
2. If you know the area you want to practice in, talk to local OD's. They'll have suggestions (usually a ways away from their own practice)
3. OD Schools
4. Trade magazines have brokers listed
#3 & #4 are overloaded with overpriced practices that aren't selling and commercial gigs with terrible hours & limited chance for advancement