Try and find a Helena Farm Chemical supplier in your area. They might have it in bulk 50# bags. There's a couple of great threads in the "muddy water" section of the forum on how much to put in the pond, and how to do it so you don't create a big pH swing.

www.hoosierpondpros.com

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).