Is there a standard chip size?
I was browsing in a local discount shop and stumbled upon trays of 100 'gambling chips' being sold for £2.99. I couldn't resist purchasing one. They are not too disimilar to the ones I've seen in UK casinos. I presume they are made of a cheaper (plastic) material, but the difference isn't striking. They perhaps weigh a little less, and have the odd imperfection.
The dimensions aren't specified, but after measuring a stack with a tape measure I reckon each chip is 1/8th of an inch thick. Colours are solid black, blue, red, green and white.
I'm not sure whether it would be worth my while to grab a couple more.
From what I can determine, the most commonly used casino chips are:
Diameter = 39mm
Weight = 11.5 grams
A stack of 25 chips = 3.25 inches (you make the metric conversion

) (you are pretty close, London Colin)
Standard colors: White = $1, Red = $5, Green = $25, Black = $100, Purple = $500 (most casinos), Yellow = $1,000 (again, most casinos)
Size and weights can vary. I would say 2% of casinos (primarily Indian) use heavier chips and their edges can be rounded instead of square. But don't be concerned about that. Once you become proficient at counting chips you can easily adjust.
If you want authentic chips (without casino logos or unique strips which you cannot buy anyway) they'll cost a lot. The average person will pay over $1 per chip. Less expensive chips that meet the size and weight requirement will cost $0.15 to $0.30 each and are more than adequate for non-casino use.
For the purpose of counting chips:
Size is important, weight is not important. Also, composition is not important. Buy the cheapest chips you can find that meet the standard size. After all, you're not weighing or handling chips, you're counting them without touching them. I recommend solid color chips as opposed to stripes on the edges. This makes it more difficult to count which hones your counting skills. I think Monkeysystem has a point about using black chips for practice.
You can find suppliers on-line by simply doing a search using the key words "poker chips".