There were lots of 7-speed freehub wheels in the past using the same 126 mm old your current wheel has. However, they are rare these days as 8/9/10/11-speed hubs are far more common and these use a longer freehub body and a 130 mm OLD. You can fit a 7-speed cassette to these hubs but you need a spacer (4.5 mm) to take up the extra width. Also, the new wheel will be a tight, but not impossible, fit into your frame unless you have it "cold set" (i.e. bent) to the wider spacing. Steel frames can be cold set but aluminum and carbon frames cannot.