There are several sources, take this one for example where you can find his stats in the photos section
https://twitter.com/messistat
This twitter user is using opta for his stats... transfermarket is great for tracking goals, penalty percentages etc but don't use it for assists. They do not use opta assists, instead they have a wider range of assists. According to transfermaket Messi has 40 assists, those 10 non opta assists will include winning a penalty for someone else to take, rebounded shots etc.
http://kahkonen.arkku.net/messi/ This right here is the holy grail of Messi statistics, you can find information on all games including friendly and barcelona under age as well as interesting stats on percentage of games scored and assisted in. Unfortunately for the assist section it includes his under 23 and under Argentina games as well which is a bit confusing as it lists 50 assists with 40 of them as opta assists, then when you choose only argentina junior level it says 11 assists with them all being opta assists which would suggest he has 29 for Argentina at senior level.
Then the spanish wiki for Messi lists him at 32 assists... I don't know ho to directly access opta stats but I think we should go with 30 as his total number.