If you are looking for a valuation for insurance purposes you need to find an accredited valuer in your area who can provide you with the correct paperwork. Check with your insurers to find out what information or documentation is required.
If you are hoping to sell, find a good quality auction house with an art expert on the team who can give you an estimate of likely sales value. Alternatively, you could approach one or more art dealers to see what you might be offered. It is usually best to email your inquiry in the first instance. Include photos of your item from both front and back and include a close up of the signature, and as much information about provenance as you have available. Paintings by this artist don't often come to auction, so it may be difficult to get a completely accurate valuation.