Dlask is probably the best place to look for a new production 12.5” barrel, I haven’t seen any for sale from Canam in ages. They pop up on the EE from time to time as well.
Generally speaking most grizzly receivers will accept factory Remington barrels, that said there has been issues with how they fit depending on the generation of grizzly you have. The biggest issue can be how the barrel indexes or fits with the ejector, it may be canted a touch or the ejector can be a touch big and not slide into the grove in the barrel. The index can be hardly noticeable, or it can leave the front site or rib at the 11 or 1 o’clock position.
It’s best to try a Rem barrel first, maybe you know someone that has one you see how or if it fits before you buy one. I’ve tried it on two Grizzly’s and both locked in tight but one had a barely noticeable can’t just past 12 o’clock, not enough that it would affect accuracy.