Of material there are two kinds, cotton and fibreglass. Fibreglass is more expensive and lasts almost forever and draws the oil up better than cheaper cotton wicks.
The only drawback is that fibreglass is an "unnatural" material, in which some magicians say it might be less powerful than something natural. (Like how some magicians swear by beeswax candles instead of ordinary wax ones.)
Of wick shape, there are also two kinds. The typical and common round sort, and the more unusual flat sort, like that of a tape. "Flat" wicks mostly come in cotton, and these draw up the oil better than ordinary round wicks due to their larger surface area. They are also less likely to get clogged so quickly with impurities. (E.g. Hoodoo condition oils.)
Wick holders are mostly made of thermal glass and can be ritually cleansed and reused once the spell is complete. To remove the soot I just use a metal skewer to scrape with.
Wicks are almost always white. But sometimes if I am lucky I happen to spot coloured wicks. Here is a blue-coloured cotton wick of which I shall be using for healing shortly.
But of all wicks, it is the flat sort that has been capturing my attention the most. Your petition can be sewn right into it with the correct coloured thread. Even better if the target has long hair, the hair can be sewn into words as the petition itself into the flat cotton of the wick with a needle.