It is very good that you are starting an online shop. I had a friend of mine who had an online shop which had a third party handling the payment system. But, later on he had his own system for the payment and had used the services of SSL Star for the purchase of SSL Certificates
that provide security in terms of the information submitted by the user. GeoTrust, Verisign and RapidSSL are some of the trusted providers of this certificate.