A couple of suggestions..
This is actually more likely to happen at PEAK times. For Orlando that is the Christmas week os be aware. And it's more likely to happen if you booked a midsize or compact. The Suburbans probably are not as risky as they aren't high demand. The truth is that they have "cars" just not cars they want to give you at the price you paid. I think only ONCE have I had a rental car company COMPLETLY out of cars.
1. Do not move away from the counter regardless of what they tell you.
2. Do not scream or yell. Cursing is also not allowed. Acting like a 2 year old being told no does not help
3. Explain that THEY took the reservation and they have a commitment to get you a car that is at LEAST the car class you reserved. If they continue to stall ask for a manager. Still do NOT walk away. You may however turn around and tell the people in line behind you LOUDLY that "they don't have any cars"
4. If you see the person next to you getting a car point that out.
5. Be polite, but stand your ground. If anyone else is getting keys they have cars.....
I do like the idea above about calling the 800# for a manager. However do NOT leave the counter. Use your cell phone.
Eventually to get rid of you they will find a car. I once made Hertz "sublease" me a car from another company
The only time this has not worked for me was L&M. They have a unique method where they "upgrade" the cars depending on demand. So they claimed they were giving me what I reserved when in fact it was several classes below (even by rental car standards) Luckily they had made me so nervous I had a back up reservation with Hertz. Walked over got the car and drove off. So if you have "low cost"