Go to any agropecuaria, they'll set you up. If you mentioned where you live, I'm sure people from that area could point you in the right direction. About pricing, I'm out of touch in that field so I honestly couldn't say.
They use the word 'paca' for hay, which is dried grass for fodder. 'Paja' is straw, the stalks of grain, such as wheat or rice, which is used for bedding. I've not come across wheat straw here. Rice straw is sometimes used for bedding, but the horses eat it too, which isn't particularly good for them. Wood shavings are more commonly used for bedding here.