Check in your local health food store or co-op for locally or organically grown lavender heads. Typically, you can purchase lavender like this in bulk by the pound, or scoop out only the amount you think you need for your project at hand.
Shop your local craft stores in the soap, candle making or fake flowers sections for dried lavender. These quantities generally run on the small size, ordinarily offering ¼-cup sized packages, which may be just enough depending on your size of pillow project.
Purchase dried lavender flowers online from a large retail website you trust or directly from a grower or nursery. This is also an effective way to compare the price per pound and shipping rates to get the best deal.
Ask aroma therapists in your area where they purchase their supplies. While they may not work with the dried lavender flower buds, their manufacturer for lavender oils may be able to provide you with dried flower heads.