While there are many sources for the balls and ball links the follow site is easy to navigate and has pictures of everything. 

Goto www.ronlund.com 

Click on On-line Store in the margin. 

http://www.ronlund.com/Merchant2/merchant.mv

In the search field put “ball links” to find  the plastic part and put in “balls” to find the metal balls. 

Any part number that starts with MA is a part from a company call Miniature Aircraft which are the black steel balls in the picture.  That run slightly larger (4.7mm) than the other manufactures (4.72 ~ 4.75mm) balls.  They will still work but an MA link on another ball will be on the loose side and a another link on an MA ball will be on the tight side.

The rods that the plastic links screw onto 2.3mm rod.  The rods come in different lengths and have threaded ends.  For this application the rod part number shown below should work.  It is 25mm long.  You can cut it down for a shorter link or look for another rod if you need a longer one.  Just put “JRP980121” in the search field to find it. 

JRP980121 - CONTROL ROD 25MM 2.3X25MM AC BC

There are two types of balls.  Ones with a 2mm or 3mm stud on them or a 2mm hole through them.  The MA balls have a metric allen socket in the top so that you can use an allen wrench to screw them in.  Some of the other studded balls have flatted or a hex base to tighten them with.  For the balls with holes through them you can use 2mm screws of your choice.

If the space in your application is really tight or the link needs to be really long contact me and I will help you find the right parts.