Since we are often asked where to buy various components or materials, here is a list of some of the commercial items we use on our rockets or would recommend. We do not have affiliations with the listed providers.
Jolly Logic: https://www.jollylogic.com/
A great range of altimeters that are very easy to use. We use both the AltimeterOne and AltimeterThree.
CoolThings Australia: http://www.coolthings.com.au
We use these tornado tubes to join spliced bottles neck-to-neck when assembling longer rockets.
CoolThings offers a great deal when you buy a box
Water Rocket Pop!: http://www.waterrocketpop.com
Mike Passerotti's PVC launcher with several add on options: http://www.waterrocketpop.com/ orders.htm
Used for launching rockets with a full bore nozzle.
This glue is a flexible polyurethane glue that sticks fairly well to PET. We use it to seal bottles when splicing. It's also suitable for attaching fins and other components to the rocket.
We source most of our electronics from Digikey. They have a huge range to choose from.
Hobby King: http://www.hobbyking.com
We use these batteries with some of our flight computers. They have a great capacity and weigh around 2grams.
E-bay has the best bargains for RC-Servo motors. They are best bought in packs of 6x or more as you will want to have a few spare when your rockets don't do as they are told.
Search for "RC micro servo" and then search for the one you want. We mostly use the 9 gram versions.