It will be hard to do online. CO is a considered a dangerous gas, and is actually explosive at higher concentrations in air, so getting it shipped will be trickier than getting an inert gas shipped.
Industrial gas suppliers in my locale sell it, but it's more expensive than an inert gas, and there are hoops to jump to through, so... eff it, inert gas it is. If I have to go through most of the steps for an insert gas exit anyway with CO, why bother? Less chance of poisoning someone with a bit of dispersed nitrogen than CO, too.