I dunno, things may have changed since I bought mine but I didn't have a lot of success finding something locally.
When I checked with Calgary Oxygen, the only thing they were willing to sell me was a 2lb cylinder. Now, I actually have and use a 2lb cylinder on one of my reactors but I have to say that refilling it every 3-4 months how do you say "sucks".
I didn't have any luck getting any of the fire supply stores to *sell* me a 20lb cylinder but maybe things have changed.
One guy at Liquid Air had the audacity to tell me that it is "illegal in Canada for individuals to own a CO2 cylinder" (I wish I was making this up). But he was tripping over himself to sign me up for a rental contract. That tells me there is a fair bit of margin in the rental business, and that should tell you it's better to own your own cylinder in the long run...
Beverage companies may be worth a go but all I got out of it was a runaround. Maybe if you knew someone in the biz they could set you up. My 20lb cylinder is fact a beverage cylinder...
Sam's link looks promising, I'd be looking at one of those maybe. Cylinder looks pretty much identical to this one:
http://www.premiumaquatics.com/Merch..._Code=co2units
And OA seems to have a bunch at reasonable prices. If I wanted another cylinder I'd just buy one of these:
https://www.oceanaquatics.com/index.php?cat_id=39