OK, so they're not incense holders... but with some holders you might well need these easy-to-light (sometimes referred to as "self-lighting") charcoal tablets, which are essential for burning incense resin granules, and can also be used for gum-based dhoop etc.
From Swift-Lite, a name you can trust for quality, they're formulated to light easily, using only a cigarette lighter or candle... and pure enough to use in traditional Middle Eastern water-pipes (also known as sheeshas, shishas or hookahs), as well as for incense.
They're just the right size (approx 33mm / 1.25" diameter) to use in most incense burners. A tablet burns for around 30 - 45 minutes, depending on conditions. They come packed in airtight foil rolls of 10 tablets.