When I was in Toronto & Mississauga I noticed a couple Walmarts, but never went inside.
Regardless, ASK an employee where their craft section is.
You'll never find it yourself unless you're very familiar with that sewing bead section.
Bring a photo as of the mesh on your phone, psp, ipad or whatever you kids carry with you nowadays.
I purposely don't ask for help on days where I exhausted, so if an employee doesn't come over to ask me if I need help, I'll leave after I take about 500-2,500 steps in the store.
Most people would consider that a waste of time, but I get in some exercise & I have a great idea of what type of items that stores carries.
Good practice of self control too in terms of impulse shopping.
Anyway, like most people have asked, take some photos of your current tanks/setups.
Post them. I can give you easily 100 different ideas on what you could use as a COVER.