My daughter's a premmie & was only 2.7kg at birth. We bought premmie clothes at M&S and H&M (their sizes seem to run small, so 0-month fit).
She didn't really wear a lot of onesies during the first month as they were too big. Instead we put her in one of those shirt with ties, then swaddled her.
As for diapers, we found pampers premmie ones at Gateway. We also used regular newborn pampers and folded the top of the front before taping the Velcro. Huggies don't fold as nicely because it's stiffer. Some Japanese brands are also smaller in size, too.