31.8 MM is the handlebar and stem clamp diameter. 1-1/8" is the stem clamp diameter on the fork steerer tube. This is the standard for most modern road bikes. Length usually run 50 MM to 140 MM depending on manufacture, model, application and all the [censored].
I prefer shopping Amazon to eBay, but both offer up some good deals. Check out the English websites...ChainReaction, Wiggle, ProBikeKit, PlanetCyclery, Ribble, Evans, Merlin, Halford's, etc. Prices are very good and shipping is free if you buy $75-$100 worth of products. Here there is WesternBikeWorks (BikeTiresDirect), Jenson USA, Nashbar, Performance, Competitive Cyclist, Colorado Cyclist, Branford Bike, ExcelSports, etc.
Bookmark those sites and shop them before you make large purchases. And remember that most offer price matching.
There's nothing wrong with supporting your LBS, but I usually build up my own stuff or wear out parts fast enough I want to shop around. I make sure that several LBS's get my cash for some expendables, food, some clothing (always buy shoes IN PERSON!) and what have you.
10 days is about the normal shipping window from G.B. or the continent. Plan ahead and delivery is rarely an issue. Competitive Cyclist just had free 2-day shipping on any order $49 and higher.