I look for a 1 MB A500+ chipRAM expansion.
A megachip? There's a guy on eBay selling brand new ones he is building himself, in the UK. His ebay username starts with Adee (I can't remember the rest of it), and he posts a new one just about every week. But, that's only for the A500.
Or, if you mean the expansion that goes into the trapdoor on an A500, there's PeteAU (on EAB and maybe here), who sells two versions, one for the A500 to make it 512Chip/512Fast, or, another for the A500+ that will route it into chip. He's on ebay, as well, or his own website is:
http://pibus.info/amiga/pa501/
