Buying roofing material for a new roof project is not easy. Browsing through catalogues and setting up your mind on some materials, textures and colors is not a guarantee that those materials are suitable for your project, therefore you should avoid making such decisions on your own and consult with a professional contractor instead.
Buying the roofing materials is definitely your responsibility, as a customer, but when it comes to purchasing them physically from a manufacturer and transporting them to the site, you can have your roofer deal with this process, if you regulate it by contract, to make sure that the materials are exactly those you pay for. Besides, some roofing contractors team up with certain manufacturers, which could get you discounts, or at least good offers, as well as full warranties.
Of course, you can also purchase the roofing materials on your own, if this is really what you want, but do not be too surprised if the roofer you contact for the installation will not be able to use them as a part of the roof system they can offer, which can make you lose some benefits included in roofing offers.