Barley straw is most effective if added autumn through spring before the algae takes hold. If the water is less than 10C it may take 6 - 8 weeks to start working but in water over 21C it may become efffective in as little as 1 - 2 weeks. It will slowly decompose and remain effective for around 6 months.
2 - 3 bales per surface acre of pond is the recommended amount so, since most ponds are a lot smaller it doesn't take much. A pond 3000 - 4000 litres should only need about 230g of barley straw. Less than 3000 litres use 120g.
Put the straw in a mesh bag or old pantyhose to stop it spreading all over the place. Attach a float at one end (it sinks as it decomposes) and put it in the pond with the straw partially out of the water. It needs exposure to water and oxygen. You could also put it at the top of a waterfall unit.
In warm temeratures, you can add another 'dose' of straw after 5 months and remove the old one 2 months later so they overlap doses. This ensures a continuous algaecidal effect.
If you live in a colder area, put the barley straw in the pond once the temperature is above 10C.
Make sure the straw is organic and free of chemicals.
Make sure you get barley straw and not hay, wheat, or other straws. Barley straw is the most effective of the lot and hay will actually increase algae growth and deoxygenate the water as it decomposes rapidly.
Do you have any farming friends you could ask for some? I would enquire at gardening centres, nurseries, or farming supply stores. Even an organic store may be able to point you in the right direction to their suppliers.