The above is the only yellow GTS that comes to mind in Matchbox size scale. Unfortunately since it was only issued in a 5-pack in 1999 it won't be the easiest to track down in quantity. Also, buying them 1 at a time + shipping, you can easily average $10-15 each which wouldn't be cost effective to use as handouts in my opinion.
If you're OK with a non-GTS, this yellow Matchbox GTS-R was recently available in stores and is just starting to be phased out with newer batches. You might have better luck looking at Dollar Generals or Family Dollars at this point as they usually run a few batches behind the bigger retailers like Wal-Mart and Target. The positives are you can find many, pay only $1 each, and if the black stripes bother you, they can be removed with non-acetone nail polish remover.
I know it's not what you wanted, but just something else to consider.
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