Rockwell print

If this is for insurance or tax write-off and you do not have an invoice:

http://www.appraisers.org/courses/

The IRS requires such a document for donating any work and your insurance company should advise you if they need a written appraisal if, in fact, you do not have an original invoice.

If you are trying to sell the piece, there may be some information on the interent by Googling Rockwell with the title of the piece. This will give you an idea of what it is selling for on the secondary market. I have little knowledge of what the market for Rockwell is these days. Because of the sluggish economy, no art other than bonafide old or modern masters that are in museums is doing that well. The print market is extremely volatile.

Right out of the gate, here's one entitled "Our Heritage." If this is your inquiry or it is a color lithograph of that quality, that's likely a retail value. However, it's almost impossible to sell a print for retail over the internet -- E Bay is the worse because they have no conception of the value of any art.

http://www.gallerydirectart.com/bej-3402.html

An accredited appraiser, for instance, is likely to appraise that piece for half of the retail value.

BTW, if you are considering buying the art, please be sure you love the artwork, can afford it and have a place for it. As I've elsewhere advised, do not buy art for investment, especially paying retail for it.