I’m about start my new vertical Offshoring for small and medium companies please suggest suitable site to get the list of such companies
Such a list doesn't exist. No one is going to hand you a list of leads that is worth anything. And even if a list like that existed, it wouldn't be as simple as calling people and asking for their business.
You need to meet potential customers where THEY are looking, at the right time, with the right offer. And for offshoring, it will be a longer sales cycle because it requires developing a significant level of trust before someone will spend money with you. If you're just starting, you don't have the track record required to win business immediately.
Your initial sales strategy needs to be different. You need to focus on a few specific targets one at a time that you're certain you could both provide exceptional value to, and that are interested in your type of offer. You will likely need to make a deal with those initial companies that is very different than what your future service offerings will be. And you want to make sure these pilot customers are willing to give you excellent references.
We have no idea what vertical you're working in, so even if I could point you to a list of companies in that industry, it's a mystery whether they're anywhere near considering offshoring anything or whom you'd be competing with.