Does the printing substrate have to be glass? If so, what size?
Obviously being in a rush can limit your options, but here are a few thoughts:

Quick solutions:


Blue painters tape (as Carl mentioned) will work directly on your heated bed...assuming it's a flat piece of aluminum with the heating element under it.  Your surface does need to be flat.
Acrylic plate will work but is best with no heat, or low heat.  PLA sticks to it.  It's easy to cut and easy to find at local hardware stores.
Scrap glass is fairly easy to find for free and it's not that hard to cut...or buy some at your local hardware store and have them cut it.  Just be careful, it's sharp. You can sand the sharp edges (wet sandpaper) to make it safer.  No, it doesn't have to cover the whole plate, but obviously covering the whole plate would normally be preferred.


Better (but not as fast) solutions:


Buy some custom tempered glass.  I like One Day Glass because they're fast and very capable.  Like Tom says, many many people print on glass because it's nice and flat and stiff.  It's also easy to clean and holds up well.  You can print on the bare glass with many materials or use various preparations like PVA (glue stick or white glue diluted with water are popular), hairspray, or others.
Touch Screen Glass: Some people like replacement glass for tablets or touch screens because it has a hardened scratch-resistant surface.  I've not tried it, but if you can find one that is the right size, it might be worth a shot.
Specialty products: There are many many print bed products out there that promise an easier/better printing experience.  I've used a few that are okay...definitely better than blue painters tape, but I much prefer my current favorite...
My current favorite is PEI. Use a 3M 468MP adhesive sheet (it handles the heat well) to stick a thin sheet PEI (also sold under brand name Ultem) on top of your glass plate.  I got this idea from Lulzbot and it's what they use on all of their printers.  It works beautifully.  Common filaments stick to it while it's hot and release easily while it's cool.  For other filaments I just use PVA like I would over glass.  For material printing tips and settings that work well on PEI beds, check out Lulzbot's site.  When it starts wearing out or getting scuffed up, I just sand it with fine sandpaper and I'm good to go for another hundred prints or so.