Top3 cards, R4i gold 3ds, Acekard 2i and DStwo. But currently Acekard 2i can not work on 3DS V4.2.0-9, so just buy r4i gold 3ds or DStwo instead. Recommend buy r4i gold from here: http://www.dsflashcart.com/r4i-gold-3ds-wood-r4i-3ds-flashcard-for-ds-dsi-3ds-p-248.html
just take a look at the description of here: http://www.dsflashcart.com/r4i-gold-3ds-wood-r4i-3ds-flashcard-for-ds-dsi-3ds-p-248.html "Pre-flashed, no need to update by yourself! Working great on the DSi V1.44 and 3DS V4.2.0-9." preflashed means it can work out of box.
As far as I known, r4igold 3ds from r4ids.cn is the first card that can support the 3ds v4.2.0-9 directly.I found some more infos about it from here: http://www.gamekool.com/r4i-gold-3ds-version-p-402.html It's listed on official site as a US reseller. Its free shipping and fast delivery impressed me.