Here are a few cheap guitar straps for Rockband, not sure but I'd assume that they would work on the Guitar Hero ones as well. All eligible for free shipping, so they'd make decent filler items.
Octopus Design $3.30
Tibetan Design $4.20
Gothic Design $4.20
The too cool to be discounted Design (Skulls $9.99)
Octopus Design $3.30
Tibetan Design $4.20
Gothic Design $4.20
The too cool to be discounted Design (Skulls $9.99)