Actually they're NOT quite as rare as you think. AGP is essentially DEAD with the advent (and shipping) of PCI Express.
Companies like Dell who sell their low end 2400 series that has a really crappy on board video solution (Intel Graphics Extreme 1) and NO AGP slot have actually kept the market alive for decent PCI solutions, along with people who want dual monitors without replacing their existing AGP card.
There are ATI and nVidia solutions available. One of the more popular ones is the FX5200 using a GeForce chip. You can find them under relabelled makers like "Mad Dog" and others at office/computer stores like Office Max, CompUsa, etc..
You can also get good PCI cards from NewEgg.com too.
Now.... to find one with TV out... that's gonna be harder and you're gonna pay more for them, unless you can find some old ones on ebay. The problem is, those old ones usually had pretty crappy output compared to the current crop on AGP. (The tv out hardware wasn't quite baked yet in the rush to get them out)
In the long run, I think it's cheaper to convert a TV into an arcade monitor ($65 for 19" and $95 for 25-27") and connect to even the cheapest video output and use Advance Mame and you get an awesome picture and more resolutions, depending on your card, than the single resolution a TV out card can give. But of course you need to have knowledge to do the conversion.