So unless you have a bunch of excess Vibranium lying around (it's a fictional element, so I doubt it), it's going to be difficult to replicate Captain America's shield. Have no fear: it's not impossible! Here are a few different techniques that cosplayers have used to build their own shields for their Captain America cosplays.
Youtuber bloodyguy9898 made his shield from a sled. In the video, he teaches you how to strip the paint from the sled and replace it with the correct colors, and add a gloss. This technique is great, and the supplies are inexpensive and readily available.
Indy Mogul used a satellite dish to create their version of the shield. Before spray painting the shield, they distressed the metal to make it look more like it did during the film. This one also has straps, so it might be easier to carry around all day (say, if you were to bring it to a con). There's a similar guide on instructables.
Finally, if you're looking for something more temporary, check out this guide by Dali DIY. They provide a downloadable PDF so you can print out a template for a cardboard version of the shield.