Well the best deal is to get them free; Check with some restaurants, bakeries. If you want to buy them, Lowes sells 5gal. buckets cheapest : about $1.80. Fabric shopping bags cost under a buck. But I don't know how long they will last. I have heard some Walmats have them (the blue ones) for 5o cents each. My neighborhood walmart has discontinued selling those.

We've a dollar store like place nearby that frequently has 3gal buckets. Our Walmart's blue bags are not a woven fabric and fall apart when wet.

Root Pouches are the answer -- Greenhousemegastore has the 5 gal for only $1.31 when you buy them in batches of 10. Superior in every way to plastic pots and shopping bags. Buckets will heat up too much and you'll burn your roots. Go with the Root Pouches:

Here is a link that might be useful: Root Pouches