Any auto parts store should be able to get stock repalcement pistons. There are several AMC specialty vendors -- try
www.amclives.com (American Performance Products). They have a lot of speed parts for AMCs. So does
http://www.classicone.com/wsc/index.htm.
Some of the parts for the AMC engine will cost more than Chevy or Ford parts, should be on a par with Pontiac, Buick, etc. In the end you end up spending about the same to build an AMC as a Chevy of equal power. The main reason is the excellent heads used on the AMC. They are arguably the best stock iron heads ever made as far as flow rate. All 1970-91 AMC heads use big valves and ports and flow about the same as Chevy LS-1 heads. 70-mid 71 has smaller combustion chambers for higher compression, but are otherwise the same as later, lower compression heads. So don't get sucked into the old "319 heads are the best" (319 is the last three digits of the casting number) myth! Buy the right pistons for the compression you want and the later heads are equal. You would need to buy a set of aftermarket heads or heavily port run of the mill stock Chevy heads to equal the AMC heads.
Some people don't like the fact that there is really only a choice of 3-4 intakes for the GEN-3 AMC V-8 (1970-91; GEN-2 is 1966-69, GEN-1 1956-66. GEN-1 shares nothing with 2/3, 2/3 are almost the same except for heads and block deck height -- most parts interchange, including heads).
Darn! Hit submit before I finished!!
As I said, fewer choices for parts. But the parts that are available are the tried and true parts that AMCers keep buying. So there are fewer choice, but those available are the ones known to work. As far as intakes it boils down to what kind of driving/racing you're going to be doing.