How can I purchase stocks in Alternative Investment Market traded (AIM) on London Stock Exchange from US?

There is nothing to stop you trading AIM stocks from the US. It is part of the London Stock Exchange, so any broker that offers you LSE access should offer AIM trading as well.

Start by checking with your current broker, if they already offer LSE access there is no point opening a new account. If your current broker does not offer LSE access you could open an account with Citi or Bank of America, as both will allow you to trade LSE stocks. If you really want to be adventerous you could try opening an internet based account with a UK based broker like Barclays (barclays.co.uk), but there is probably no point unless you intend to do a lot of trading. Once you have provided identification, confirmation of your address, cash and signed all the right papers you are good to go.

Please make sure you understand what you're buying. In the UK the stocks that trade on AIM are small capitalisation and the volume can sometimes be a little thin. As a group they are high priced at the moment, large cap stocks in the FTSE30 are on a much lower valuation. Finally, you are buying stocks in a foreign currency and movements in the UK Pound/US Dollar exchange rate will afect your profit.