Then that leaves the three options already talked about all involves money. If you want to use your Tx.

Use a reversing Y chord. This device reverses just one of the servos plugged into it. A mixing Y chord will not do this it is for an entirely differently purpose. An even more expensive matchbox may do this as well I don't know and never had a need for them.

Use a similar hitec servo one side (this is not a desirable choice)

Special order a matching servo reversed from factory default.

If it were me I would simply reposition the servos when they should be but I realize you may not want to do this.

John