I occasionally will need a glass lid for a piece of bakeware and rather than carrying the actual item around with me I take a piece of cardboard and make a template of what I need, Does not matter if it is round,square,oval,has pour spouts or not, I make a template and write the measurements on the template, I mark the outside diameter/perimeter and the inside diameter/perimeter and I always carry a little tape measure and folding ruler with me.
Whenever I am out I will stop at local thrift stores,Goodwills,flea markets and yard sales, even if I have to buy the complete piece just to get the top as long as the price is right I will do it.