I don't think you can get the p-nitro exclusively. According to my experience, a mixture of nitric and sulphuric acid works well and you get the p-nitro as the major product, together with some o-nitro toluene. Chromatographic separation of the two compounds is very easy

As Irene says, using the NO2+ electrophile, you always get a mixture of o- and p- isomers with the p- isomer predominating(especially at lower temperatures) because it is formed faster. However, you may be able to do much better than 55% by Thallating your toluene first. For this you require Thallium(III) trifluoroacetate in trifluoroacetic acid solution. As a result of steric crowding a much greater proportion of the +Tl(CF3COO)2 electrophile goes -p to the methyl group (Olah, Iwao and Lin quote 86% see Proc.Natl.Acad.Sci.USA Vol74, No 10, pp4121-4125 Oct 1997). If you the salt out the -p-Thallated derivative with saturated sodium chloride and then treat this with your nitrating mixture you will have a beeter yield of -o isomer. Remember though that the final positioning of a a substituent in aromatic rings is subject to both kinetic and thermodynamic control. The para isomer is usually formed faster with o-/-p directors, but the temperature should be kept below about 20 centigrade or you start to incrase the proportion of ortho isomer.

I agree with Irene Sanchez Molina and Paul also. But, probably you may get answer in electrochemistry.