[mykeal]

Nipples are designated numerically by the thread designation, length and then cap size. The most common are:

Metric thread designations -
M5.5 X .9 (5.5mm or 0.216" major thread diameter by 0.9 mm thread pitch or approx 28.2 threads/inch)
M6 X 1 (6mm or 0.236" major thread diameter by 1 mm thread pitch or approx 25.4 threads/inch, ISO coarse thread spec)
M6 X 0.75 (6mm or 0.236" major thread diameter by 0.75 mm thread pitch or approx 33.9 threads/inch, ISO fine thread spec)
M8 X 1 (8mm or 0.315" major thread diameter by 1 mm thread pitch or approx. 25.4 threads/inch, ISO fine thread spec)
M8 X 1.25 (8mm or 0.315" major thread diameter by 1.25 mm thread pitch or approx 20.32 threads/inch, ISO coarse thread spec)

SAE thread designations -
1/4 X 28 (0.25" or 6.35mm major thread diameter by 28 threads/inch or 0.907mm pitch, UNF thread spec)
12 X 28 (0.216" or 5.486mm major thread diameter by 28 threads/inch or 0.907mm pitch, UNF thread spec)
1/4 X 32 (0.25" or 6.35mm major thread diameter by 32 threads/inch or 0.794mm thread pitch, UNEF thread spec)
5/16 X 24 (0.312" or 7.94mm major thread diameter by 24 threads/inch or 1.058mm thread pitch, UNF thread spec)
5/16 X 18 (0.312" or 7.94mm major thread diameter by 18 threads/inch or 1.411mm thread pitch, UNC thread spec)

1/4, #12 and M6 are rifle and pistol nipple sizes; 5/16 and M8 are musket nipple sizes (in general, but sometimes...)

Length is specified in inches or mm, depending on the thread spec, and cap sizes are generally #10, #11 and musket.

The 12X28 and M5.5X0.9 specs are pretty close to each other and can be mistaken if just casually observed. 5/16X24 and M8X1 are also. You really need both SAE and metric thread and bolt size gauges, and they need to be in good condition to be absolutely accurate on what size nipples you need. If you can't borrow one from a local mechanic or machinist they are not too expensive, but get both SAE and metric gages.

All that being said, Track of the Wolf has the best selection of sizes that I've come across, Thunder Ridge is also very good and has a superb cross reference chart. I'd start with those guys.