Various manufacturers make armbands with generic cases/pouches on them (the pouches tend to be closed–that is, there is no see-through feature). You could check at a store like Best Buy, Radio Shack or Frys.com. I believe I’ve seen this from DLO or Belkin (or maybe Kensington).
And here’s one from Meritline.com, at a whopping $1.99 shipped (!) (and I’ve seen it on “special”/promotion at Meritline for under fifty cents, believe it or not, including shipping–Meritline constantly/hourly is running small item promotions of this type, which can be fun to follow, as listed at its website launch page and in periodic emails you can register for). It can be used on the included armband or attached to clothing (or anything ese) via the included caribiner clip (the armband and clip probably make it worth the price alone). It seems to appear larger than it is–the listed measurements make it seem just big enough for the Clip. Meritline tends to ship out of Asia, and so shipping can be 1 - 2-1/2 weeks, but I’ve found it to be dependable.