Have you tried Aer Lingus or Iceland Air or any of the non-legacy carriers? You may have to settle for a connection somewhere in lieu of a direct flight. I personally don't mind a connection because it's always a price/convenience trade-off. Where are you flying from in CA? Obviously, if you're near enough a large travel market like LAX, then you'll do fine with airfares...if you're in a small city, then it will be more difficult. I wouldn't use a travel agent just to book flights, it's just like booking flights in the US. If the fares seem unusually high, it may be because you're looking out too far into the future...so remember that they're not static and change all the time.
If you pick a spot on a tour that's not in peak season (middle of summer), you should be able to get a decent deal. Think about what flexibility you have - and if you do have some, then I would use it to tour at a time when fares aren't at their peak. I would make sure that there are spaces available on the tour before booking any airfare. If you're taking an RS tour, there is more flexibility in making a date change than any flexibility an airline will grant you (most likely none, if you bought a non-refundable fare).
Also, many people come a day or even number of days earlier just to settle in and get over jet lag before the tour starts. It's not necessary, but it could give you time to acclimate and it may help with airfare costs if you have a few days wiggle room (either arriving early or staying a day or more late).