Can I get a 20 pound CO2 cylinder filled with Helium?

I recently got a 20 pound CO2 cylinder and was wondering if I would be able to get it filled with helium gas.

It is certain it is physically possible to put some quantity of helium into an empty CO2 cylinder, but I don't think that's the way the gasmongers usually do things.

I think the usual way it works is that you go to your local gasmonger, and say I want some kind of gas, let's say helium, and then the gasmonger offers to rent unto you a tank in which to store the helium, and there is some yearly or monthly fee for renting the tank.

I think that's the usual way they do things.  Probably the easiest way to find out would be to ask your local gasmonger, "Hey, can you fill this CO2 tank with helium." 

Don't be shocked if he or she runs shrieking in fear, or crosses his or her fingers to ward off the bad luck or evil spirts you must be carrying with you.
It's true that  there is a certain kind of tank intended for each kind of gas, based on characteristic vapor pressure (if the gas is liquified), chemical compatibility, etc.  Anyway, the idea of putting some kind of gas in a tank not intended, not approved and sanctioned for that kind of gas....

Well you can understand how some people might find the idea troubling.

Also regarding helium, I don't think you'll be able to liquefy it at the pressures a CO2 tank can safely withstand.  It would just be pressurized, to whatever pressure is the maximum pressure you can put in a CO2 tank.