What is a good vegetarian source of umami flavour?
The Umami information Center has a list of Umami-rich foods along with natural concentrations of glutamate.  I've copied some of their list below (included some meats for comparison) in case the link disappears (concentration number is mg glutamate/100g food).  There's also some information at the above link about how to prepare the foods to maximize the umami taste.

Food/Concentration:


Beef/107
Tuna/188
Nori/1378 (not a typo)
Tomatoes/246 (the riper the better)
Fresh Shiitake Mushrooms/71
Dried Shiitake Mushrooms/150
Enoki Mushrooms/22
Soy Beans/66
Potatoes/102
Sweet Potatoes/60
Chinese Cabbage/100
Carrots/33