If you are willing to do some work, then check your area's Craigslist free posting. This time of year there are tons of free river rock postings. If you do not want to dig up someone else's landscaping... I agree with the two other postings, quarrys or feed stores. Good Luck
In our area, river rock can be bought cheaply at feed supply stores. They won't carry as many fancy types and shades but they will have some nice looking rocks. You can also try Lowe's when they offer sales on this type of thing.
I would recommend a local rock quarry...there are two where I am located and I am able to buy large quantities at a cheap price. If it is for a small job, as said already lowes and home depot sell bags of rock