Where to source large cheap plastic/ paper bags to put sold items in

[Tillyboo]

I'm doing my first Xmas Fayre in a few weeks and have some large cushions that I've made and so need to source some large bags to put them in if I sell them.
I need at least 18" x 18". Failing finding any, has anyone got any other ideas as to what I can put the cushions in? (not Tesco carriers)
Plastic or paper, I'm not fussy. I've looked on the internet and you generally need to buy at least 100!
Is it the done thing to provide a bag ? It's got to be more professional I suppose but I don't want to wipe out any profit I might make on forking out for packaging

Are you anywhere near Watford?
There's a very cheap wholesaler called Bowmans.
You'd probably have to buy 50 or 100 bags but I bet they'd be so cheap you wouln't mind.
They don't do mail order - you have to call in person. There's a sign saying you have to spend £50 but if you pay cash they won't mind that you don't.

Thanks Fortyplus. Too far for me unfortunately so I'll keep searching