Re: Good place to aquire this year's game elements?
Check with your regional representative and/or the FLL tournament directors that ran your local season. They may be willing to part with the fields and pieces from those events. Particularly if you make a good case that by doing so the FLL will gain increased exposure.
Failing that, bricklink.com is the online marketplace for Lego pieces of all sorts. You may not be able to get everything you need from one store. Remember that color is not of great importance in making field models, so if you have to make that trailer out of non-white parts, it's not a big deal. Look around for the best deals in parts. Usually the used ones are cheaper and not of much inferior condition.