Petite Suits

Austin Reed
A perfect black trouser suit in a petite fitting; 100% wool with a simple classic design in sizes 6 to 18.
Wallis
A nice selection workwear in sizes 8 to 18 including tailored jackets with skirts and trousers to match. And they're not all black!
Marks & Spencer
Affordable suits with trousers leg length 26", skirts and dresses.
Dorothy Perkins
Dotty P's have a small selection of skirt suits starting at UK size 6 for most items. Good for basics in neutral colours for your working wardrobe. Prices are in the cheap and cheerful range and you can get free delivery on your online order if you click on the link above.
Next
Pages of suits with skirts and trousers plus workwear separates. Some are more 'fashiony', while some are more sober and serious. There is also a brave departure from basic black as some grey and brown items are also included. Sizes start at UK6.
Karen Gillam
Smart yet fashionable suits in sizes 6 to 18.
Principles
Principles have a small selection of their petite smart clothing range available as separates generally of the classic basics type (i.e. black & grey). Sizes start at UK6.
Amazon.com
At present Amazon.com only ship clothing to the U.S., offering a range of Petite brands including, Coldwater Creek, Nordstrom, London Fog, J Jill, Eddie Bauer, Liz Claiborne, Anne Klein New York and Lauren by Ralph Lauren. However Marketplace merchants may ship internationally, check individual details.