we have petsmart but they don't carry it. I tried a different "blue" shampoo but didn't really find that it did much. My last try was a clarifying shampoo (from the pet head line) which is the best I've found so far but not perfect. i'm still on the hunt.
I THINK "clarifying" shampoos have a different purpose... to remove built-up product from the dog's (or human's) hair. As such, they can be too harsh for regular use, as they can strip natural oils as well as conditioners from the coat.
I think Lucile talked about a human product for people with grey hair (meant to remove yellowish tinges) that worked well for her. Maybe she can remind us what she used, and maybe that would be easier for you to find. It might even have been one of the (too many IMO!!!) Pantene shampoos. If she doesn't see this and respond, you might want to PM her.
The other option is to just use any regular shampoo you like and add a TINY splash of regular laundry bluing to it. This is what we do for white horses, because you need a LOT of shampoo!!!
Just be careful... use a TINY amount and work up, or you could have a blue dog!
A little goes a long way. When I was a kid, we used too much bluing when washing a white pony who had some manure stains. We got the manure stains out, but he was blue for weeks!