Affiliate Buy It Now Button

There are several. Only you can decide which one would be best for you. It depends on what you are looking for in particular.

If you are looking for one that counts the outbound clicks, you could search the plugins directory specifically for ‘affiliate‘, however if you want something very simple, you don’t even need a plugin all you need is to add a link to post and attach a class to it:
eg

<a href="http://wordpress.org" class="buttonlink">WordPress</a>

Then you can add some css rules to a child theme or custom css in jetpack to style these links.

eg:

.buttonlink {
   border-top: 1px solid #96d1f8;
   background: #65a9d7;
   background: -webkit-gradient(linear, left top, left bottom, from(#3e779d), to(#65a9d7));
   background: -webkit-linear-gradient(top, #3e779d, #65a9d7);
   background: -moz-linear-gradient(top, #3e779d, #65a9d7);
   background: -ms-linear-gradient(top, #3e779d, #65a9d7);
   background: -o-linear-gradient(top, #3e779d, #65a9d7);
   padding: 8px 16px;
   -webkit-border-radius: 10px;
   -moz-border-radius: 10px;
   border-radius: 10px;
   -webkit-box-shadow: rgba(0,0,0,1) 0 1px 0;
   -moz-box-shadow: rgba(0,0,0,1) 0 1px 0;
   box-shadow: rgba(0,0,0,1) 0 1px 0;
   text-shadow: rgba(0,0,0,.4) 0 1px 0;
   color: white;
   font-size: 16px;
   font-family: Helvetica, Arial, Sans-Serif;
   text-decoration: none;
   vertical-align: middle;
   }

.buttonlink:hover {
   border-top-color: #28597a;
   background: #28597a;
   color: #ccc;
   }

.buttonlink:active {
   border-top-color: #1b435e;
   background: #1b435e;
   }

You can make a customised version of this CSS at http://css-tricks.com/examples/ButtonMaker/