Source for loose 3mm ball bearings needed

That's about gerbil size, isn't it? You could raise some. They breed really fast.

-- Ed Huntress

McMaster as always, search "bearing balls", catalog page 1124:

High-Precision Bearing Balls E52100 Alloy Steel? Bearing-quality chrome-finished steel balls are through-hardened. Rockwell hardness is C60-C67. Inch sizes conform to ASTM A295. Balls are Grade 25, with a diameter tolerance of ±0.0001" and a sphericity of 0.000025" (unless noted).

E52100 Alloy Steel? Metric Sizes Dia., Pkg. mm Qty. Per Pkg.

3 100 9292K33 5.94

McMaster p/n 9292K33

If you're talking about an THK/IKO sort of linear bearing, those are often selective assemblies. Graded balls are used to set the preload and accomodate manufacturing variation in the rail and car.

Make sure the balls don't alternate in size. Sometimes you have a big/little/big/little/... arrangement.

Wes

-- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller

If it's a really low-tolerance situation, bicycle shops should have 1/8" BBs in stock... grade 200, usually a couple bucks per hundred, maybe twice that for chrome steel grade 25.