The urethane belting in the US is typically made by Eagle Belting (a division of Fenner Drives). The site
http://www.fennerdrives.com/eagle/ has all of the Eagle choices on it. Additionally, there are a number of local distributors that sell Eagle belting. Unfortunately, the only non-reinforced flat belting that Eagle makes comes in that lovely dayglo orange.
As for seal the belting, the site listed above also has welding kits. Many of the kits boil down to a simple soldering iron with a flat piece of 1"x1" 18- or 16-gauge steel attached to the tip. The two ends of the belt are heated with the plate (one per side of the plate) and once the urethane gets to a molten temperature, you quickly slip the plate out and mate the ends together. This could be a two-person job or you could get one of the fancy kits that helps you out with some clamps.
Although it doesn't help you much, our team started using Eagle Roundthane this past year and everybody loves it (no welding required). Unfortunately, it doesn't come in a flat form.