Try N.C. Ordinance Inc. or Homestead Parts, both out of the U.S.
Most retailers up here can't even be bothered to stock an assortment of shotgun beads, so it's unlikely you're going to find grip caps.
OP, the grip caps from NC Ordnance are resin casts, very similar to many of the original factory grip caps, on specific rifles.
They ship to Canada.
They use original grip caps to make a mold, then just produce them as needed. Takes about a month from the time of ordering to receiving.
I just purchased three from them, which are clones of the broken and missing grip caps from some rifle stocks I'm working on now.
They aren't cheap, but if you're not set up to cast your own or don't have an existing grip cap to make a mold from, it's a simple and time saving option. They're quite nicely finished.
I believe they also have metal grip caps. Go to their website. It's easy to traverse, and they have dozens of different caps.
Brownelle's carries cast metal grip caps, which may be Pewter, but I'm not sure. They appear to have some of the same caps as NCO, and they're similarly priced.
I like grip caps on wood and even composite stocks. They add a "classic, finished" touch.
Your other option is to visit or get in touch with a local gun shop or gunsmith and inquire about grip caps. Some will have new ones in stock, but most don't.
Some of them salvage the grip caps from broken stocks, which I do myself. Most are a close fit, but some caps can be excessively large or small
and it hurts to trim a regular cap.
Guntech's solution is a good one if you have the tools and talent to do the job, and if you like the look of a wooden grip cap.