Make a 100% guarantee. Something like "we'll have the gear you want, delivered when you want, or your money back AND we'll give you credit to rent the gear on us at a later point to make up for it."

Internally, make it your policy to go above and beyond to make sure even if there's a huge corporate event, that you still have the necessary gear for everyone else that has paid. Your job is to deliver and have the gear. Making it happen in unusual circumstances is part of the business.