The best I can recommend is an atmospheric model maintained by NASA that includes dispersions up and beyond your altitude of interest. The model, GRAM, provides dispersion estimates across the globe. It works well above 60000 ft. However, at altitudes below that it does not capture local terrain effects from mountains and coastlines. To resolve that, NASA and the USAF have used thousands of measurements from balloons and other sources to build Range Reference Atmospheres (RRAs) that they use to verify requirements for launches and landings at a few select places across the United States (such as Kennedy Space Center).
Finding atmospheric data above 250000 ft is difficult. Back in the 1980s, the American space shuttle orbiters began skimming the high altitudes of the atmosphere for thousands of miles during their return to earth. Reviewing the flight data, NASA discovered density shears or "potholes" in the drag accelerometer sensor data that couldn't be attributed to any vehicle attitudes. The flight data ended up being incorporated into the GRAM model dispersions.
You don't need to incorporate the model directly into your simulation -- you can run GRAM stand-alone to generate atmosphere profiles that your simulation then reads.