It's not looking good these days (especially for the full size cart), the following threads are fairly active but you'll need to keep an eye on them on the off chance someone finds a legit site with stock.
Better to post/check those than create even more threads (there are plenty more older ones in the GBA section).
http://gbatemp.net/threads/ez-flash-iv-is-being-discontinued.339309/page-14
http://gbatemp.net/threads/buying-a-gba-flash-cart-in-2013.341203/page-15
http://gbatemp.net/threads/need-to-get-ez-flash-iv.351475/page-4
http://gbatemp.net/threads/should-i-overpay-for-a-ez-flash-4-or-go-with-alternatives.352143/page-3
The DS lite sized carts are still available here (posted by FAST6191 in another thread):
http://shop.01media.com/info.asp?ProductID=33276
They can be modded to fit in a GBA with a bit of effort.
You could also use an EZ-Flash 3-in-1 in the DS lite (in combination with a DS flash cart to load the ROMs from):
http://www.realhotstuff.hk/EZ-Flash-3-in-1-Expansion-Pack_p_15.html
That's the DS lite version, there is a full size GBA cart version also.
The 3-in-1 has no memory card slot so needs a DS and DS flash cart to load ROMs onto it.