You can purchase an SSL certificate from third party vendors such as Comodo (www.vpcart.com/virtprog/instantssl1yr.htm), Thawte (http://www.thawte.com) or Verisign (http://www.verisign.com/).
If you already have your own SSL certificate, then you can configure VPCART to use it.
VPCART also supports shared SSL, which is where your web hosting company provides an SSL certificate for you.