I get them from Servall. They have a whole selection of them in a display by the parts counter. Since I live only 3 miles from store, I have no problem just replacing them as I use them. You can call and order / As I just replace them as I use them so I'm not interested in buying them in bulk.
For truck stock, I keep a dozen each of 1/4-1/4, 1/4-5/16, 5/16-5/16, 1/4-1/4 90 degree, 5/16-5/16 90 degree, 1/4-1/4 quarter turn valves. I cut the valves in the back of refrigerator lines as an option of running down the basement. Easier to service. The 90's are really good in tight spots when replacing lines around compressors. Allows you to get extra space away from hot lines to keep plastic from getting brittle and cracking again. I also carry a few 3/8-3/8 and a few 3/8-1/4 guest connectors used when installing some insta-hots. I keep an additional 3 of 1/4-1/4 and 5/16-5/16 in my toolbox for a quick line repair and I don't have to run to truck for part. I also keep a 2 foot piece of 1/4 and a 2 foot piece of 5/16 in my toolbox.