The standard approach is to execute a non-disclosure agreement with the potential consultant(s). I have many times requested quotations from consultants on projects containing proprietary technology and have never had an issue with the ideas being stolen. Assuming the non-disclosure agreement was executed (perhaps as part of the request for quotation) the consultant would risk a lawsuit if they tried to steal your idea.
If you are in the US, you could file a provisional patent application, but you do need to know what you are doing to make it useful and consulting with a patent attorney or agent is recommended. Also, not all aspects of intellectual property with regards to computer applications are covered by patents. There are also potential copyright and trademark issues so consulting with a lawyer is a really good idea.