Some pretty amazing Mellotron music for free download can be found here. A fellow named Mike Dickson owns a Mellotron M400, a mixer, some computer with recording software, a measure of musical ability, and apparently a certain amount of time on his hands. So, he recorded All-Mellotron versions of Holst's "Mars, The Bringer of War" (inspired by King Crimson's assaults on the tune nearly 40 years ago), swell interpretations of Adagios by Albinoni and Barber, Bach's famous "Air on The G String",
For more authentic orchestral sounds, the Milwaukee Symphony Orchestra has free samples of their recordings (generally complete movements from larger works) available for the asking here. Click on the "MSO To Go" picture, surrender an e-mail address, and broaden your horizons. Also recommended.
*For playing along at home, try these variations on a Ground