A used set of B110 GX mirrors just popped up on eBay recently, you're going to have to pay more than $300 USD though. This might be a good buy for another member on the forum looking to build a GX clone:
http://www.ebay.com.au/itm/3117156126 ... geName=STRK%3AMEBIDX%3AITThat eBay seller is also selling a set of used 510 mirrors for $350 USD which might be what you're after:
http://www.ebay.com.au/itm/DATSUN-510 ... d9cbd3:g:52QAAOSwNsdXTEdJ