Could someone point me to a suitable heatsink for an LM3876 amplifier? Or, even better, a heatsink that I could attach two chips to? Perusing through Mouser's catalog was a bit overwhelming.
If you start with the power management calculation you'll see that, unfortunately, there will be little choice left.
Let's assume you want to use the LM3876 at its full power, 56 W. The datasheet gives 150 °C as absolute maximum junction temperature (page 4), and a junction-to-case thermal resistance of 1 °C/W (same page). That means at 56 W the case temperature shouldn't be higher than 94 °C. If we assume an ambient temperature of 30 °C we have a difference of 64 °C left for 56 W, so our heatsink should have a maximum thermal resistance of 1.14 °C/W. That's low, but it's the absolute maximum; if you want to play safe and only allow 125 °C junction temperature this would become 0.7 °C/W. For the two amplifiers on one heatsink that would be 0.35 °C/W, which requires forced cooling. These numbers also assume you have a perfect thermal contact between the LM3876 and the heatsink, which won't be the case.
So first start with determining how much power you want from your amplifier. Note that with a pair of decent speakers (90 dB/W) 2 x 56W will give you an unhealthy 120 dB SPL. You can't do that in your living room without suffering hearing loss.