I don't know if you really need 99%, but Sigma offers 98% pure Pyr-SO3. Please find the link attached.

http://www.sigmaaldrich.com/catalog/product/aldrich/s7556

Dr. Marcus sigma grade still gives low reaction yield. We have to prepare Py: SO3 complex freshly and used it. Yes we got up to 99.5% assay once. Precision is less and varies between 96-98%. We have to wash it will water so drying now become a major problem. We had dried with Toluene, Hexane.

If you use it directly fresh after preparation, you could dissolve it in organic solvent and add water-free Na2SO4 to get the rest water out, filtrate it prior usage in your reaction. I frequently used that to dry my organic compounds I synthesized.

Sorry Dr. Binner, this material is very sensitive to all the solvents so can't dry as usually organic compound we do in our lab. In any solvent it will break with in 10 min. As you suggested dissolve it in solvent and dried over sodium sulfate will not work here. But we can co-distill water with Toluene or Hexane in rotary evaporator. Still  at the time of distillation in rota evaporator, there is a still chance of breaking of the complex.

You may try,Adding 1 mole of dry pyridine over crushed SO3 , 1,001 mole, in 4 parts of CTC, dried , moisture < 0.005 %, Crystalline product collected is  washed with littles ice -water and then dried over P2O5 in a desiccator, overnight