Go here:
http://www.micromark.com/decal-paper.html
They have clear and white decal film. If your decal has no white then you can use the clear, if the decal image has white you'll have to use the white paper and carefully trim off the unwanted parts. There are a few "Alps" printers out there that can print white but most of us use an inkjet printer and some a laser, just make sure to get the paper for your type of printer.
The "fixative" they sell is nothing more than a can of Krylon "1313 Clean Satin Finish" that you can get at a local store. It can also be used to seal tissue which is what people are using when they say they used "Crystal Clear".
The Mico-Mark directions say to spray this on the decals straight away then let dry. I found that when trimming off the excess paper the ink was breaking up and flaking off which ruined the decal. I had all but given up when I saw John Oshust (over at the Virtual Aerodrome) getting stellar results with the same paper. Based on his advise I let the ink dry for 24 hours or more, a couple of days if I am not in a rush. Then I dust off the paper with a tack rag that does not leave a residue then spray with the crystal clear fixative and let dry for another day or two.
Oh yes, and do not touch the paper on the side that gets printed on, the oil from your fingers will leave a lovely finger print in the decal. Also, print on plain paper till you are satisfied with the decal then switch the paper type to "glossy photo" or similar then run the decal paper through.
The decal sheets may seem spendy, buy when you consider that for maybe $2-5 you can make your own decals and create a model that no one else has.