Red neckers are available in many LEGO® sets especially in current train sets. They are also available from the online Pick-a-Brick or from Bricklink* where it is listed as a "bandana". Bricklink has new and gently used neckers in every colour ever produced.
ShirtsThere are several choices for shirts produced by LEGO® over the years. Perhaps the simplest and most effective is the Star Wars ® torso (Rebel Technician) shown above as part of a basic uniform. The two patch pockets make a nice uniform shirt but does require a necker to be added. Some of the torsos which you can see at the top of the page actually come with neckers or bandanas as part of the printed design (ie: Professor Trelawney torso). This feature is especially useful if you wish to equip your Brick Scouts with a backpack as bandanas and backpacks are not compatible.
If you are feeling more adventurous you could even design your own custom decal such as was done by Red 5 Bricks. A great tutorial for creating your own decals or labels can be found @ Classic-Castle.
HatsThere are fewer choices for hats than for shirts. The least expensive and most commonly available is the green cap but you are welcome to choose another colour more appropriate such as blue for sea scouts or red which is used in such countries as the United States. There are plenty of other outdoor hats available as well if you want something different. Find what works best for you and your pocketbook: equiping a whole troop with expensive headgear may be too much.
* Note: BrickLink (BL) is an independant service provider. Links to BL pages are for information purposes only. Pick-a-Brick is a service available from LEGO Shop@Home.