You can get decal paper at most full line hobby stores. There's one brand that's pourous enough to accept inkjet ink, but i understand it's expensive. If you get clear paper, which is commonly availble, you'll need to do a white painted background.
Now, as to making multiples:
Get the image you want, downloaded, scanned, home made, whatever into your photo editing software. Clean it up and edit it as you se fit and need. Save the image where you can find it as a .jpg.
Open your word processor program and insert the image as you would any clip art. (With MS Word, it's InsertPicturefrom file and chose the picture you want.
Once in the document, resize as needed. When you're happpy with the size, right click it and chose copy, then use the paste command to make as many copies as you want.
Save the "decal sheet" document to a floppy or cd and take this to a local office store that has a color laser printer and have them print a copy of the document onto your decal sheet. That should cost about a dollar.
To be safe, seal it with MicroScale decal film and apply as you would any other decal.