R4i Gold (r4ids.cn)
+fast menu (thanks to wood kernel)
+great homebrew support, no need to use hbmenu to run some
+save states
+multiple save file slots
-most expensive of the R4i cards today
R4i 3DS RTS (r4i-sdhc.com)
+cheaper than R4i Gold
+multiple save STATE slots
+in game guide (read your .txt guide during game)
+the team is still active, a kernel update came out last month
+built-in skin editor
-menu is clunky
-some homebrew requires hbmenu.nds to run
R4i Dual Core (r4isdhc.com)
+cheaper than R4i 3DS RTS
+-clone of R4i 3DS RTS, so the same pros and cons
R4i Dual Core (r4isdhc.com.cn, r4isdhc.in)
+usually the cheapest R4i cards available
+-hacked wood kernel, fast and seems to work well but I did not do extensive testing
-no save states or ingame guide
As for where to buy, check out the Online Stores and Purchases part of the forum for suggestions.