is there another master cylinder that could fit on my 2007 chevy optra LS wagon?? My cylinder is a 'european fit' from what I'm told...

Why not get the original part? that is a fairly new car and the parts should be available- is it because it's expensive? is it because you live on an island? It is not common operating procedure to go looking for substitute parts on braking systems-

Not only not common but potentially dangerous

I cant afford it new... single mom and I cant find one at any scrap yard because the cylinder for the LS wagon is different than the other cylinders in the non-LS models

good luck- you have run into the ever-present problem that faces poor people with automobiles- they cost a lot to keep running- whatever the new one costs, save for it, borrow for it, brakes are a safety issue, and a used master cylinder will have the potential to kill you and your children- cars are not a joke- or a human right- I am poor and when I can't afford to get my car fixed, I have to walk and ride the bus- fortunately I live in a city where that is possible- some cities (Houston, LA) are impossible to get around in without a car- there will be a heavy reckoning if the economy takes a downturn in those places- as a society, we have not addressed our unfortunate reliance on the almighty automobile- but you can plan for your own life- find a better job, start your own business, move to a smaller city, buy a piece of farmland- staying poor in the middle of a big city is a losing game-