Yes, you can buy an SSL certificate from any vendor.
An SSL is issued based on the so-called Certificate Signing Request (CSR), which is a block of code generated on the basis of the contact details you enter for that SSL.
If you have already bought an SSL from another company, you can download/copy the CSR code, the Private Key, which was generated with it, then add them along with the SSL from TightHost Control Panel -> My Domains -> SSL Certificates.
If you have any issues, you can also contact us by opening a ticket from the Help Center -> My Tickets in your hosting control panel.