Yeah, 6 shot is what I use for squirrel hunting.
The market is awful right now. With everyone panicked over the upcoming elections, kung flu and violent mobs, "pickens' are slim" and is a terrible time for first time gun owners or people just now buying a new gun that they don't already have a supply of ammo to feed it.
If 6 shot is all you can get, is better than nothing though if that's all you can find. As I told a friend several years ago, low brass is better than no brass.
Just be aware that you are very limited in its' useful range because the small pellets lose most of their momentum rather quickly in flesh and don't tend to go deep enough to stop a determined bad guy or penetrate deep enough to reach the vitals.
One last ditch option for you may be to try to find some 00 pellets in the reloader section and perhaps open the top crimp and dump the birdshot out and replace with the equivelant weight buckshot and then try to crimp the hull back. But that still doesn't answer whether the gun will cycle the shells. Only firing some will answer that.
During the great depression, many didn't have access to buckshot and slugs so they used round creek pebbles or even lead muzzle loader balls. Or even wax slugs. Though, if you're unfamiliar with reloading, those options also come at a risk, but are some options to consider if you absolutely must.