28-8-16 water soluble lawn fertilizer

I have an EZ-FLO fertilizer tank lawn irrigation sprinkler system that can accept any water soluble or liquid fertilizer. I want to buy a 28-8-18 fertilizer for this tank.
Can anybody recommend some good places to buy this in bulk online? I want to buy enough to last me for a few years. The sprinkler company is charging me $30 for a 5 pound bag, what a rip-off!
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anyways, i found the best place is LESCO... lesco.com
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