I will be assuming that you are a member of a university or an academic society hosting the conference and you don't have access to a conference organising group (whether in-house or private).
First, and rather importantly because it is often overlooked, I would recommend that you review whether the university or society has restrictions on the receipt of sponsorship monies. For example, some groups have strict rules about receiving money from tobacco companies or pharmaceutical companies because it would jeopardise existing or future grant opportunities. You have to be careful about this. You don't want to receive GBP5,000 from Marlboro Cigarettes and find out that your lab is now ineligible to submit grants worth millions of pounds over the next two years.
Second, if your society or university has a marketing unit, I would contact them. They have great contacts and often receive unsolicited offers of support from other groups or charities. They might be able to provide leads, introduce you to corporate sponsorship officers or even take on the role of fund raising from your hands.
Third, if this were a recurring event -- say, it's an annual meeting -- then I would pull up the program from the previous year and note the sponsors. I would seek advice, too, from the previous organisers.
Fourth, there's no harm in writing to potential sponsors. For small companies, you would write directly to the CEO or managing director. For larger companies, you might try writing to the director of sales and marketing. You would lay out your needs and invite them to support you. In general, you will need to work out a sponsorship agreement or contract laying out these details. Some companies have such agreements already in place, or your university of group may have its own. Regardless, you need to be aware of the statements and conditions in these agreements.
Finally, most sponsors do this for advertising. Some will want some outrageous things. In one sponsorship deal, we had to mention the name of the company five times during the night of the Gala Dinner. Try as he might, the MC had a hard time working in jokes about a new colorectal cancer drug while people were queuing for the buffet. Some will want time to conduct "continuing education sessions", which are generally closed door meetings about an area related to the company's area.
Good luck.
