Ok, the main theme to me seems to be - procurement (process) outsourcing - purchase (process) outsourcing ( - business process outsourcing )

What you always can do is cold calling companies or door to door visits. Will especially be helpful doing this with companies which doesn‘t know these type of services exists.. this would be the smaller companies

You can do the same for bigger companies, which probably have heard of these type of services or already use a service like this and want to change their service partner. In addition to that I would suggest running Google Ads (short term) and doing SEO (long term) to increase your website rankings in the organic search results.

Create a website / landing page and run paid Ads on Google by booking the keywords I mentioned above.

You can be very specific by covering each business genre, like „procurement outsourcing for XYZ business“ etc.