Where should I get parts for 71 Chevy?

[ZZSPIRAL]

I've had this 71 short bed fleet side with a stock 402 since 1889. It runs like a Swiss watch, but begs to be restored. My excuse was it needed a bed, but I found a beauty 2.5 years ago, so excuses no more. Where do you recommend parts?

Also, I need to put in new duct. It does have AC. Do you recommend the OE, or is the aftermarket better? It's just duct. Not like you'll see it.

Thanks for any input.