You probably won't be able to purchase a pre-made converter cable but you can easily make one with a 4 pronged male plug and correct 3 prong female, along with some #10 gauge wire with 3 wires inside it(available by the foot at Home Depot). Note finding a female end(other than an outlet you'd need to mount in an outlet box) may be hard. I mean you can go the plug, wire, single outlet box, outlet, outlet cover, route but an easier solution might be to just cut(or preferably take apart and remove if it's not a molded plug) the existing EVSE plug and wire on the one you want.
Your EVSE will have 2 hot wires and a green ground. When wiring on the new plug connect the hots to the hot, ground to ground and the neutral lug on your plug won't have anything connected to it. No safety issue as EVSEs don't use a neutral, just two hots. Well if it's a 120/240v EVSE it will use a neutral when plugged into a 120v outlet but in that case your outlet neutral will go to one of the hot wires going into the EVSE, doesn't matter which one.
Last edited by jjeff
on Sat Apr 08, 2017 8:36 am, edited 2 times in total.
2012 SL purchased used 2/'16
2013 S w/QC purchased new
Juicebox Premium 60a L1/L2 EVSE, Ebusbar 16a L1/L2 EVSE
'12 EVSEupgrade'd 20a L1/L2 EVSE, '13 EVSEupgrade'd adjustable 6-20a L2, 6-13a L1 EVSE
Zencar 13, 20, 30a L1/L2 portable EVSE
GE Durastation 30a