You can go online and buy white or clear transfer paper - I have done both many times in the past with custom decals I needed to make. They have the sheets suitable for inkjet or laser printers, both in clear and white. Just create your sheet on your computer and print using the proper sheet based on the design colors and the printer you have.
http://www.decalpaper.com/
The last set of decals I made using the decalpaper stock actually worked better than the decals supplied in the kit. Here are some tips:
1. If you need a lot of white on the decal, you have to use white paper. Just note that you will need to cut each decal out separately, so it can be difficult if you need to minimize the background, tiny designs, etc.
2. Follow the directions on the sheet - pretty darn easy
3. You MUST use a spray sealer on the sheet after printing. If you do not buy the decal sealer spray, you can use a VERY LIGHT coat of clear acrylic. Too much spray and your decals will not shrink and bend very well when using the decal set solvent. Too little spray and your design may start to rub off when trying to position the decal.
4. If you only need the white area as a small spot or so, it might be easier to paint the white spot on the model and then use a clear decal sheet, especially if the trimming around the edge of the design is problematic. Clear lets you just cut a general shape and slap it on.
Obviously, other normal decal rules apply - make sure surface is smooth and non-porous, use decal set, seal over decal when done, etc. I use semi-gloss acrylic and brush a really light coat on the spot I will apply the decal to give a smooth surface to bond. Then I go over the decal with the semi-gloss again to seal or you can use matte finish at this point, if that is the look you want.