I use a Carradice Camper for commuting and I absolutely love it. It is simply the best option I have come across in all my years of commuting by bike.

...The only problem I have with the bag is toting it around with me when i am not on the bike. Till recently carrying the bag with me meant tucking it under my arm but I am hoping to find a better option.

Last week I made a feeble attempt at being smart and took some old toe straps and fed them through the "d rings" on top of the bag to make a kinda-sorta carrying handle....but it looks funny it doesn't work all that well.

Ideally, I would like to have a shoulder strap for this bag but I am not sure what to look for or where to look.

I use a clip on nylon webbing strap that came with my panniers. It has a buckle that lets me adjust the length of the strap. These are typical and I see them everywhere, though I'm not sure where you can buy one on its own. Usually they come with bags.