Not great news - couple on ebay from European sellers, including Analogic, who've been around forever it seems.
But they won't ship outside Europe.
Maybe ebay trading rules are an issue here, not sure.
You could try phoning Analogic (5 hours ahead of Eastern time, remember) and asking nice. They do have a few, and do sell things outside of ebay. They are old school sellers who still have bits and pieces of Amiga kit, mostly they fix old Mac nowadays. They do seem happy to trade worldwide direct from their store. Orders outside Europe get big postage, but also 20% discount as no VAT (sales tax) applicable.
http://www.analogic.co.uk/Don't get them mixed up with Analogic US, totally different people. Tell them an old pal of Mr Gupta sent you (he was the founder, IIRC). If you end up talking with him, say Pat McDonald from Format says hi.
EDIT: Here is the ebay listing, but I don't think you can buy through that and get shipped to USA.
http://www.ebay.co.uk/itm/Amiga-500-1MB-Memory-Upgrade-Card/182424118218?_trksid=p2045573.c100033.m2042&_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20131017132637%26meid%3Dd5d20598fbda4ccb987e0580c3a074aa%26pid%3D100033%26rk%3D2%26rkt%3D4%26sd%3D112284410985
