"Simple groups of Lie type" by R. W. Carter, a table after Section 12.4. But there only the values of $C_{\alpha\beta11}$ are listed. An explicit form of commutator formulas inside $U^+$ is given in Table IV of "Chevalley groups over commutative rings: I. Elementray calculations" by N. Vavilov and E. Plotkin, see the picture below.

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/CVNnA.png