You can get dedicated SSL certicates from many providers like RapidSSL, GEO trust, SBS and many other... Basically they are installed on a secure server to communicate securely with its web clients. If the SSL certificate is not installed on the server then any information sent to a website can be intercepted and viewed by anyone.
Main advantage of having dedicated SSL is to gain trust of your potential buyers. Some customers may feel uncomfortable to put their credit card details on a website which has different secure URL. If you want your customers to put their credit card details on your shopping cart then dedicated SSL is much better. Dedicated SSL also shows that the SSL Certificate is in your company name and dedicated to your website.
Yes building trust of your potential buyers is one of the important factor to establish your business, you should enable necessary Security features to provide secured payment transaction to your buyers, it will also help you to gain confidence of your buyers.