I bought some 4/0 recently from City Electric Supply, paid close to $5/ft. Check with local Electrical Suppliers.
For cut lengths, I also order from www.shopwirenco.com
They have good prices on wire & shipping.
Should be an easy find. 4/0 is common in residential & small retail services. We use MCM (or kcml) sizes up to 2000, for energy production & distribution.