I didn't buy any new clothes for any of my children in their first year. I bought all the clothes second-hand on e-bay or from jumble sales or the the NCT Nearly New Sales.
It's only when my children reach the age of 3 that I start buying nice new clothes. At least at that age they last the year!
For my ds born in March 2007, all the items were from a fabulous aquaintance who gave me 5 bin bags full of clothes. I kid you not, there were 106 vests in the 3-6 months age group! She does have 2 boys but still. I will be forever grateful. She effectively gave me about £400 worth of free clothing and I barely know her. I can assure you, it was the best Christmas present I've received for a long time. Some of the clothes were either new or barely worn.
You could also put out a shout on freecycle or ask your local NCT for details of NNS or whether they'll put a shout out for any spare clothes from other NCT members. Members like myself regularly pass on wearable clothes to others free of charge.
I found that in the first year, you don't really need much. My children spent most of the 0-3 months in 0-3 months sized sleepsuits and vests, so I would get lots of those. You don't really need many outfits for the 0-6 months. For 6-12 months, lots of socks but be warned: they are easily lost so do not spend a lot of money on them.
Also dungarees. During the 6-12 months stage, your lo will learn to crawl. Dungarees, with poppers on the inside legs are best. Tescos and ASDA sell bargain new clothes for this age group, so if you can't find them second-hand, go down to TESCO and ASDA for excellent value new clothes.