Fixed Quantities

Has anyone figured out a way to sell fixed quantities online?  For example we are a personalized stationery company and only want customers to purchase our personalized note cards in fixed quantities of either 25, 50, 75, 100 or 125. We could just set up quantity pricing to give the discounts for higher volume purchases, but that would still allow a customer to order any quantity they want, as opposed to the fixed quantities we sell.

For lack of a better solution we have had to set up each quantity as a seperate item.  Therefore we have to set up 5 items (for each quantity) for "Margarita Time" notecards, for example.  And have to do this for each of our 1000+ other designs/products.