You might try to get sponsoring a similar way that you get it for a research:

• If it is a technical conference with a industry exhibition you could get your sponsor to exhibit and offer yourself to represent the company.
• Some conferences offer discounts for presenters, so you could present a paper/poster.
• If the subject of the conference is closely connected to your research project, take the fee from your project budget.
• Send a junior to attend the conference, he might get a discount.
• Stay at home and order a copy of the conference proceedings.
• The more expensive a conference is, the smaller is it's scientific value (according to my limited personal experience).

Many thanks dear Ingo Riess :)