500+ domains for one time and for one registrant, i think that will be done by domain reseller according to my previous experience, direct purchase with market price won't be that wise.
if they are .in i can help (PM me for other tlds too, i may be able to help you) for .com or other tlds for cheap.. try GD as said
I suppose any domain name registrar will be happy to offer you good discounts for such bulk domain name registration
I can arrange a special prices for you from my godaddy reseller account, so that from your account you'll be able to buy/renew/transfer directly from buy rates, especially for some extensiojns you can not find a competitive price. If you are interested let me know.
You can do a few things.. Pay market value for them, try and find a coupon and hope the shopping cart of whatever registrar you use doesn't break, contact them by phone and see if they can give you bulk rates (though their discounts will most likely only be pennies) If you are planning to pay for them all (or a good portion of them) at once (you can take however long time you want to register them afterwards, or even register them all at once) I can get you them for $2 USD/year and up, depending on what extensions you want, with free Whois privacy through my registrar. You can pay by CC, PayPal, WU, basically anything. Good luck with your project no matter where you get them from. PS. To those others interested in buying, we currently only do bulk orders. You can PM me for more information.
In what TLDs are you going to invest? Also can you PM me at what price are you looking to pay per domain? Have a look at www.OrderDN.com Thank you
Which TLD? If they are .com/.net etc I would contact a registrar directly and ask them for a bulk rate. Make sure they are actually ICANN accredited and not just a reseller (since they accredited ones will have lower rates).