Unfortunately I don't think there's a free space MIB. I believe there's size and used space, then free space is determined based on that. It's not even a statistic that SolarWinds keeps histroically. Looks like it's calculated on the fly based on the last polled values and calculated values for percent used. You'd probably have to come up with a custom report that would give this to you then put that in a view. Something like:
Select
Nodes.Caption as Server,
Volumes.Caption as Drive,
(100 - VolumeUsage.PercentDiskUsed) as "Percent Free",
(VolumeUsage.DiskSize/1024/1024/1024) as "Disk(GB)"
From Volumes
Join VolumeUsage on Volumes.VolumeID = VolumeUsage.VolumeID
Join Nodes on Volumes.NodeID = Nodes.NodeID
Where VolumeUsage.DateTime Between (Getdate() - 6) AND Getdate()
AND Volumes.Caption LIKE 'C:%'