Revenue Canada is the question. Non resident taxation on sale of business interest or real estate?

Sale of shares in a business in Canada, I am told will result in the witholding of almost half of the purchase amount and I don't know who to ask for a method of reporting to claim this excessive portion back when I have no Canadian tax ID. Is anyone aware of the process or should I be contacting Revenue Canada or an accountant or an attorney? Thank you for your input.