I can easily find t-shirts for other major harmonica brands, but not Hohner. I also get stickers from another brand when I buy a harp, but not Hohner. I love the 30 day free online school coupon included with Hohner harps (when registration is open).
My advice: offer (sell) several T-shirt designs in a few colors and all sizes and give away or sell stickers. This marketing shouldn’t be an afterthought and neglected. It builds brand loyalty and will provide a significant return on a tiny investment.