Savage made the 24x series from the 60's until the 80's. They came in a range of calibers:
22lr/magnum over 410
222rem over 20g
357/357maxi over 20g
30-30 over 20g
Eventually they licensed these off to Valmet, who made a .308win over 12g. Expect to pay $300-$900 for the savage rifles. I'm not sure what the Valmets go for, never got to use one yet.
Baikal Russia is making, or made the IZH series which included 223, 308, and .30-06 over 12g. I hear they're built like tanks and ugly, but decent accuracy. The one thing I don't like is how the 12g is over the rifle barrel, seems weird. Epps had these for sale about a year ago, and there's a Canadian distributor who just replies: "Not come to Canada bye!".
CZ makes or made combo rifles and I've handled them a couple times. Typical CZ craftsmanship, almost too nice to take in the bush but not too expensive, just like everything else they make.
Remington I believe had some a while back but stopped making them. The American forums talk about them once in a while but I've never handled one before.
Tradex I believe has some weird caliber combo rifles for sale. If only they'd chamber one in 7.62x54r over 12g, people would probably buy them here.
What you should really do is read the google books online version of the gun bible. Use the forums or google to identify the model numbers you're after, then fine tune your research with a published book.
EDIT: You may seriously want to look at this rifle:
http://www.shop.tradeexcanada.com/content/suhl-drilling-12ga-12ga-and-8mm
12g, 12g, and 8mm that would be absolutely badass, and it's $369.00. I strongly recommend you snap this up before I get paid.