Southeast Asia is packed with would-be Buddhists, travelers on a real spiritual mission espousing lives of detachment from material desires. These folks usually walk away with just the "one suit, two shirts, trousers, and a tie package" when they leave Hoi An. Shopoholics wander the streets in a daze.
Hoi An is a silk mecca. The quality and selection are the best in the country, and you'll have more peace and quiet here during a fitting than in Hanoi. Silk suits are made to order within 24 hours for about $35 (£19); cashmere wool is $45 (£25). There are countless shops, and the tailoring is all about the same quality and fast. A good way to choose a shop is by what you see out front -- if you see a style you like, it'll help with ordering. Make sure you take the time to specify your style, down to the stitch (it can come back looking pretty cheap without specifics). Try any of the shops along Le Loi; to recommend one in particular would be like recommending one snowflake over another. The tailoring is very fast but not always great, so plan to have two or three fittings. Be choosy about your cloth, or go to the market and buy it yourself (Hoi An Cloth Market is at 01 Tran Phu St.), and haggle. It's not a bad idea to bring an actual suit or piece of clothing that you'd like a copy of. Get measurements from friends and relatives for good gifts.
Note: Unless otherwise noted, most stores listed below are open from 8am to late evening (around 9pm), but hours can and do vary.
There are also skilled cobblers who make custom shoes at affordable rates. Find them near the market on Tran Phu Street.
Tran Phu Street is lined with art galleries and good pottery and woodcarving vendors. Along the river, lots of places sell blue-and-white ceramics. However cumbersome your finds are, like those lovely Chinese lanterns, shopkeepers are masters at packing for travel and to fit in your luggage, and will do so before you've even agreed on a price or decided to buy. Haggle hard.