A local agronomist suggested checking here for referrals to small quantities of barley in the Adams Co area:
Arendt Hay and Grain
474 Christ Church Rd
Littlestown PA 17340
Barley straw is often most effective when added in the fall or early winter, not after an algae bloom is underway. You can read more about use of barley straw here http://extension.psu.edu/natural-resources/water/ponds/barley-straw
The biggest factor in controlling algae blooms is controlling nutrients (nitrogen and especially phosphorus from fertilizer, manure, large numbers of wildlife, septic systems, etc.). Also make sure that what you have is algae and not an aquatic plant. The Penn State Extension pond management website is here: http://extension.psu.edu/natural-resources/water/ponds and has a section on aquatic plants and algae.