This is the question I had, and I got lots of answers across different threads so thought I'd consolidate those here as it will hopefully be helpful for other people looking for premature baby clothes.
For someone with a 3.5lb baby, it was so hard to find clothes for when he was first born and in the NICU (hospital), and even as he got bigger.
Supermarkets
They all have a limited but good range of premature and tiny baby clothes. Mainly pink, blue or white and the more basic type of clothing, and we bought loads of their sleepsuits and vests (Tesco and Sainsburys. I believe Asda has even more, but we don't have one locally). It's not the sort of stuff you'd buy as a special gift for a friend but good for everyday wear.
Sizes seem to start from 5lb, and they don't have any specialist clothes for babies in hospital.
Mothercare
As you'd expect, Mothercare has some nicer tiny baby outfits, more expensive than supermarkets, but more trendy too I'd say. I was surprised though that they had nothing for the tiniest babies in hospital either.
Next and Gap
Both have some nice clothes for a 'Tiny Baby' which really means 5-7lb babies.
Noppies
This is a trendy Dutch brand that we found and loved for Tiny baby clothes (4-7lbs ish). Loads of choice and really cool clothes, we spent a small fortune with them! Check them out here (I promise I don't have shares in the company!!): Noppies Tiny Baby Clothes
Clothes for babies in an incubator
Finding these clothes was the hardest, we didn't have clue what to buy, our nursery was stocked with normal newborn clothes and none of the clothes I've listed above would have been suitable for our little'n either. This was the most stressful time for us, so spending time online shopping wasn't our priority and we were getting really frustrated until we Googled Premature Baby Clothing Buyers Guide and there were quite a few! :)