Elias - I've been using an app called Density Alt+ on iphone. There is a free version, and I don't recall the difference except that the paid version was well worth the few dollars.
On my iPhone at least, this app can use the phone's built in barometer to measure pressure, and then calculates density altitude with that and other inputs. It'll attempt to retreive those other details from the nearest weather station, or you can enter them manually. Temperature is the most significant detail other than pressure, and you should be able to estimate that close enough for most reasonably long range shooting. (i.e. estimating within 5 degrees F is plenty good enough for most 1,000 yard shots).