Here in Vancouver we have JL Aquatics, a small Canadian outlet that's growing. They specialize in marine but also have plenty of things that could also be used for freshwater. Their prices are the exact same in-store than they are online. Up the street from Big Al's, their prices beat them every time by about 25%.
While J&L may beat Big Al's, they both overcharge outrageously on any item that might be found elsewhere. Big Al also charges up to 220% for an item on the Canadian site, compared to the same item on the American site.
For things like plant nutrients, substrate, lighting, co2 supplies, and pretty much anything that could be used by any industry or hobby (including pond supplies) other than fish, you can never do worse than going through any "pet" or "fish" retailer or etailer.
My favourite is substrate. It is sold in larger bags for cheaper, for use in ponds. Sand can be purchased for use in many industries and applications, but "aquarium sand" is extremely expensive (I don't mean live sand).
I also found that HQI ballasts are significantly cheaper at hydroponic stores, than purchasing from a "pet" or "fish" shop.
We get ripped off everywhere we go, really. The one thing I found we do get for cheap is plants. Plants for ponds are really overpriced, particularly lilies, compared to what we pay for them.