The pinch a penny in Oviedo on sr 434 always has dowflake it seems. I buy it for my pool as well, 50 lb for about $35.
I have never seen the dead sea magflake around here, but I think it is pretty reasonable if ordered online. There are some links floating around the chemestry forum for an online retailer. It still comes out to less than a buck a lb even with shipping if I remember correctly. I managed to buy some of another RC'er who had some extra so never had to order it....
I think you want to avoid the magnesium chloride that is mined from the great salt lake area - I remember reading that there are more impurities in it.