Finding just the right piece of music for each production is only possible when you have a huge selection to choose from. The best way to go is usually with an affordable online library that you can search and purchase only the tracks you need. However, a few companies offer collections at such low prices that you can't go wrong purchasing them as they will pay for themselves very quickly. Digital Juice and Footage Firm have some bargain-priced music collections that are great quality, and Stock 20 has a good collection as well, especially if you get it while it is on sale. Through the end of January, they are offering their whole royalty free (pay once and use it forever) collection for just $249 and if you purchase before midnight on the 5th, you save an additional $50. Might want to check it out and see about adding it to your toolbox.
