The ornamental design for a support for sofa, as shown and described.
1.1 is a perspective view of my support for sofa;
1.2 is an elevational view thereof;
1.3 is a top view thereof;
1.4 is an elevational view of a front end thereof; and,
1.5 is an elevational view thereof from a rear end opposite that of 1.4.
The broken line showing of part of the support for sofa in the figures is included for the purpose of illustrating environmental structure and forms no part of the claimed design.