Motherhood Maternity has a nice sale for mothers to be and new mothers: buy 3 get 1 free on nursing apparel. This includes bras, tops sleepwear and accessories.
This is for regular priced items only (they say), and your free item will be the item of lesser value. All items must be within the same product category. I tried it out on this¬†Padded Cup Underwire Full Coverage Nursing Bra which is on sale for $24.99 (reg $29.98) and when adding 4 to my cart, the 4th one became free.