These are some of the most well-known online sellers of maille supplies (or at least the ones I have information on).

Happy comparison shopping!

The Ring Lord - My favorite source. Rings and wire in over twenty varieties of materials, including sterling silver, 14k gold-filled, stainless steel, copper, and anodized niobium, titanium, and aluminum. I recommend ordering saw-cut rings for jewelry. Also offers tools, kits, scales, and samples of finished maille.
* Wire gauges are in AWG for 20 and smaller, SWG for 18 and larger. IDs are in fractions of inches.

UrbanMaille - A highly recommended site with sterling silver rings and tools. Rings are saw-cut and tumbled. The kits are an excellent way to learn a weave.
* Wire gauges are in AWG. IDs are in millimeters.

GoldenMaille - Saw-cut and tumbled 14k gold-filled rings and refill packs to match UrbanMaille's kits.
* Wire gauges are in AWG. IDs are in millimeters.

Spiderchain - Saw-cut rings in sterling silver, gold-filled, and anodized niobium.
* Wire gauges are in AWG. IDs are in fractions of inches.

Metaldesignz - Saw-cut rings in a variety of materials, including sterling silver, 14k gold-filled, enameled copper, stainless steel, titanium, and copper. Features twisted enameled copper rings. Also offers kits. Prices are given in Canadian dollars.
* See the ring information page for wire gauge specs. IDs are in fractions of inches.

DeCordene WireWorks - Rings and wire in a variety of materials, tools, and kits.
* Wire gauges are in SWG. IDs are in fractions of inches.