I bought a .550" round ball mould from them several years ago. It is very nice quality. Not cheap, but it comes with handles installed. The .550" size was listed with their standard offerings, but I think they made them to order. However, I recall it didn't take long for them to fill the order.
One service unique to Moose Moulds, as far as I know, is that they give you a choice of how the sprue cutter is set up relative to the ball cavity, either as a tangential cut-off (like a Lee mould) or with a visible sprue, like a Lyman.
I've heard the company is having some issues right now. I haven't ordered from them recently. They were good to do business with when I ordered my mould.
I wish they would make a .525" or .526" round ball mould. They have a few relatively unusual sizes, but nothing in that range.
Notchy Bob