The following web portals can be used to tender for public & private contracts:
Scotland
Public Contracts Scotland gives free access to contract opportunities with Scottish Local Authorities, NHS Scotland, the Scottish Government, Agencies and NDPBs, Higher and Further Education and Emergency Services. You can also register to receive e-mail alerts.
Commonwealth Games Busienss Portal
UK
Supply2.gov.uk provides SMEs with access to contract information from a growing market which has an annual spend of £175bn on goods and services.
Suppliers who have registered on the site receive a daily email alert notifying them of new government opportunities, relevant to their business, within a local location of their choice. For many small businesses gaining access to relevant contract opportunities is a lifeline for stability and growth with the public sector making good customers and traditionally paying very promptly.

Competefor.com is a UK-wide tendering website for both public and private sector contracts.
Further Informaiton
These links will take you to further information on the Scottish Government website: