My 7.25 is toast - where can I get a upgrade?

No matter what you upgrade to, an 8 1/4 or 8 3/4 you need to give lots more info. What gearing, bolt pattern, size brakes, pinion size...you get it. No matter what you put in it you'll need driveshaft work or replacement. Guys have parts fairly close to you or ebay is always an option. I'd run a "wanted to buy" thread.
In any case I wouldn't replace that 7 1/4 with another 7 1/4 unless you really need to have it.
Here's a couple ebay links. One guy builds them to order but pricey if you're on a budget.
Good luck.

Mopar A Body 8 3/4 Rear end | eBay
Mopar 8 3/4 8.75 Complete A B E BODY Rear end COMPLETE assembly POSI Drum brake | eBay