Maybe try Carvin PA Speakers?
I use their 15 inch woofer--it's a Celestion, and sounds o-k--nothing to write home about. Was reasonably priced @ (I think) $69 when I bought it--they go "on sale" a couple of times a year.
It will take everything a Combo 300 will give it, at least until the speaker protection kicks in...I just watch the light on the amp, and if it starts to get absolutely pegged, I back off the volume a little. I could probably kill it with the amp if I did something stupid, but as long as I play sensibly, it does fine, no distortion. Not a particularly "tight" sound either, but it works for me.