I have purchased that exact switch in the past from Home Depot.... I'm not saying they still stock that same switch today but they did at one point several years ago. It's worth a shot....
Edit:
I assume your switch is momentary spring centered, correct?
The switch below is very similar but it's not spring centered. You would need to manually switch it back to the centered position, otherwise it would stay switched in either direction. Depending how quick you need it, this would probably get you by until the correct switch can be ordered.
Visit The Home Depot to buy Gardner Bender Toggle Switch GSW-16
www.homedepot.com
Apparently HD no longer stocks the momentary version of this switch.
Edit again: (lol)
I don't know where you live, or what stores are near you, but I just looked and my local ACE Hardware store has the correct switch in stock.
Gardner Bender has a versatile toggle switch family with multiple position configurations. Flat terminal heavy duty toggle switches meet the standards of today automotive, marine and industrial applications and are great for electrical connections.Find the TOGGLE SWTCH20AMP HVYDTY at Ace.
www.acehardware.com