A note of caution - don't buy on price alone. You want to know where it's from. For example:
The place I buy from specialises in sheet materials. They buy direct from the manufacturer, in volume. It comes on huge pallets, 50 or 60 sheets all bundled together, and goes onto huge steel racks. When I order it, they take it straight off the racks with a specialised lift, and load it straight into my van (or onto one of their trucks if it's being delivered).
The place I bought from previously bought it off another company, which in turn bought it from a company like the above. It had been loaded as above, but then unloaded from the truck (probably by hand) and put somewhere in a timber yard. Then it was loaded onto another vehicle, taken somewhere else, and unloaded again. Then finally loaded onto the van that delivered it to me.
By the time it arrived it had been rained on, and one sheet had actual boot prints all across one side. It was useless for what I needed it for, and it ended up all going back.
The reason I bought through the above chain was that I assumed (wrongly) that the main company wouldn't deal with me, so ordered it through a local timber merchant. After all the hassle, I went direct & found that they were in fact very happy to deal with me. I buy all my sheet materials from there now. They deliver to me for free on orders over £100, but they're perfectly happy for me to order one sheet of 6mm MDF if I want, as long as I collect it. I have a couple of named contacts in the office, and they know me by name when I call. They know that I'm trade, but also know I'm never going to buying more than a dozen sheets at a time.
Now, regarding grading - most of the birch ply I get is beautiful. Although it's graded as B/BB, a lot of it could possibly pass for A grade. I think the reason that A grade isn't commercially available is not because it isn't produced; it's because it's not economically viable to sort out the A grade sheets from the rest as they are produced. If I were to buy 10 sheets, I could pretty much guarantee at least 9 of them would be flawless. There may be one that isn't so great, but could still be used with a bit of thought in the cutting.
How many sheets do you need? Your options may be different if it's 2 sheets, 5 sheets, or 10+ sheets...