I cook a good bit of salmon. All of my salmon is grilled using cedar planks. Frankly, I would like to try Alder planks but I don't know of a reasonably priced source.
I have used this company for alder planks. They sell seconds, which are fine for me since I am burning them up anyway. I bought 30 planks (5 in by 11 in) for $26.00. Shipping is free for orders over $35. http://www.outdoorgourmet.com/wood-grilling-planks
+1 for Apple, particularly if brined first. (We like to add salt until an un-boiled egg floats, then add about a cup of Brown Sugar - pepper it a bit before tossing it on.)