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.
Yarn: Premature babies have very delicate skin and can often be allergic to wool; therefore it is best to steer clear of garments made with a wool mix. Acrylic yarn is more appropriate when knitting for preemies. Ideal yarn to use for preemie knits is baby double knit or 4ply yarn. Most preemie patterns will use these.
Early Angels: This is a site based in the U.K set up to help people to knit, crochet or sew clothes, blankets and other keepsakes for premature, low birth weight and sadly stillborn babies. They have a wide range of free knitting, crochet and sewing patterns accessible on their pattern page.
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.
The charities involved usually have free knitting patterns and advice on their websites so do not be worried by all the details as you really will be able to find a garment to suit both your knitting skills and the babies needs