Nintendo states that the 3DS is SDHC compliant, (The HC here means "high capacity") which means that any SD card that is SDHC compliant should work. Almost all SD cards that are larger than 2GB are SDHC cards and should work fine. There's a little logo on the card that looks like a stylized SD with the letters "HC" underneath. SDHC cards have a maximum size of up to 32GB.
SDHC-compliant devices can also use "standard" SD cards, but the max size for a standard SD card is 2GB (although 4GB cards existed prior to SDHC becoming the dominant standard), so there's very little chance that you'll want/need/find a larger standard SD card than the one you have.
SDHC cards come with various "classes" that give you a rough estimate as to their data read/write speed. In some applications, a device will require a minimum class rating. For instance, digital cameras that capture video might mandate a Class 6 card. Nintendo doesn't specify a minimum performance rating, so any class rating should be fine. Class 4 tends to be the most commonly available.
Many companies sell SDHC cards, and any reputable brand should work. Sometimes if you buy from eBay or overseas resellers you'll end up with a rebranded card or one that is marked with an incorrect size. However, pretty much any electronics retailer in your area should stock multiple brands that are all roughly equivalent. Kingston and PNY are two easily recognizable brands. Many brands also offer a lifetime warranty against defects on the card, much like other types of memory.
SD/SDHC cards also come in multiple form factors which are all compatible with normal SD/SDHC card slots if you have the correct adapters. MicroSD is a common format used in many mobile phones, so you can generally find them cheaply in a variety of shops. These cards will work fine in the 3DS as long as you have a MicroSD adapter. Usually MicroSD cards will come with the adapter in the package, but double check if you go this route.