If you go to your GP and mention tiredness and being a vegetarian, s/he should check your B12 levels. I'm a veggie and was having similar symptoms to yourself - turns out my B12 levels were dramatically low (120 when they should have been in the 2000 level, sorry no idea of the units here). Further investigation revealed that I have pernicious anaemia and now have regular injections of B12 and no tiredness now !
You can buy B12 tablets but you really need to get your GP to check your levels properly, to determine an appropriate solution for you.
Of course, it may be that you are anaemic (lacking iron in your diet) but that is easily obtained from green vegetables, eggs and other foods, or you could take a supplement.