There all lots of materials that you can use some for structure such as wild willow, dogwood, holly stems, de thorned wild roses, as well as maluabl weaving materials such as clematis, wisteria , virginia creeper, long leaves such as iris etc It is very easy to get started however it is not just about heading out with a set of secateurs. If you are not sure what you are doing please be cautions before you head out, some plants can at the very least cause a irritation and some much worse so if your not sure there are some great plant identify apps for smart phones and a trip to the library for a good book is always a good option.
Family and freinds gardens are great sources for materials also social media sites often give you the option to add a shout to ask if anyone is trimming back and would mind if you collected them.
Things to consider before foraging
The foragers code applies here and relates to common land or private land that you have permission to access and forage on , and apply to collection for personal consumption only and similarily to foraging for food etc ,and means that you should only take what you are going to use without damaging the plant,
We often think about private land however it is also advisable to check if there are any local by laws that apply on council owned land and other places such as land owned by the National Trust etc . For more details and as a starting place on this subject the Woodland Trust have a great guide which explores the legislation as set out under the The Wildlife and Countryside Act (1981).