AgilityEvents.net is probably the easiest place to find premiums for just about any venue. You can search on one or more venues and one or more states.
Clean Run requires that the information be entered by the users so it may not be as comprehensive as the sites that mine the other sites to get trial listings. Clean Run is a good place to find non-trial events like seminars and matches.
Dog Events Online is probably the most ambitious. For a small annual fee, it will store you and your dogs information and generate entry forms for you. It offers online entries for trials that are accepting them and agent entries for all trials in its database. Unfortunately, DEO has fallen on bad times and may disappear.