One top brand that manufactures this accessory is Knit Pro, and this particular manufacturer supplies it chart keeper to retailers around the world, making it easy to get hold of. Knit Pro also has a number of other accessories that knitters might find useful, ranging from sock blockers to needle sizer.
Fastenings: Plain flat half-inch buttons are best to use for fastening. Avoid nylon and metal fastenings, as they get very hot under the incubator heaters. Do not use Velcro as a fastening, it is very scratchy on the hook side and also damages knitted garments when they are laundered. Ease of dressing is vital for preemies and as a general rule it is best not to use ribbon as a fastening as ribbon can often be fussy to tie on a garment so small.
Size: Don’t put too much importance on trying to perfect the size of a preemie item of clothing. There is a great difference between premature baby sizes and so an item of any size should be suitable for at least one baby. As a general guide premature babies head circumference is roughly the same as their chest circumference. The average premature babies chest measurements are 8”-14”. However clothes of all sizes are needed for preemies, so no matter how big or small your item is it will most likely fit at least one baby.
One of the first accessories that will become invaluable for you as a knitter is the knitting bag. This type of accessory does not only provide the perfect place to store your unfinished projects, your yarn and your knitting needles, but it will also allow you to transport your materials with you without worrying that they are going to become damaged.
The charities have a lot of information on their websites, below is some general advice to help you get those needles clicking.