Hmmm, I've never heard of it in bulk. The PG tea I know is a combination of red raspberry leaf, strawberry leaf and nettle leaf (along with fennel and alfalfa, I think). If you could find all those in bulk, you could blend your own.
I second the Compleat Mother. Our local natural foods co-op will order tea in bulk -- there are a few brands which offer pregnancy teas, the one I think of is Traditional Medicinals, I think. My sister owns a natural foods store and she sells loose herbs so you may find a place close to you that would do it. Many co-ops are locally owned and operated and totally willing to work with you about doing bulk orders and getting discounts, etc.