Due to popular demand, here are my sources for top quality CGA nuts, nipples, and in-port filters.
For chrome plated brass and stainless steel CGA nuts, nipples, and filters, go to this link:
Once there, on the left you will see a screen you can scroll down. Scroll down till you see "Accessories."
"CGA Parts, Tube Fittings,
From this page you will be able to order components. You can also request that they send you the components via USPS Priority Mail flat rate box, which will get to you within 3 days. It will also save you some money to request this shipping option.
For brass 3" CGA nut and nipple, you can go here:
Again, you may be able to get these parts cheaper, but the ones I listed above are top quality. I use them myself on all my rigs.
The chrome plated brass and plain brass CGA nipples (stems) come with a pre-installed filter. You do not need to buy one.
The stainless steel nipple (stem) does not come with a pre-installed filter. For this option, it is recommended that you buy an in-port filter ($13 on the listed website). A filter is especially important if you use a steel CO2 tank--less important if you use an aluminum one.
Hope this helps.