Honestly
amazon is probably the cheapest. I was going to buy it but ended up getting the smart ato with a tank I bought, but
Amazon was the cheapest I found it. Keep in mind though, with that Ato I do not believe it comes with a pump from what I recall, so thats like another 20$. Still cheaper than a lot of them though.
There's 4 stores to check when purchasing things in my opinion:
Amazon- Usually as cheap as the other places, don't always have it, but free shipping with prime
Marine Depot- Lots of stuff, high free shipping minimum
Dr. Foster & Smith- Runs good sales frequently, 50$ free shipping
Bulk Reef Supply- Good if they have it, or need unbranded supplements and stuff, high free shipping minimum.