|Le Carra seems to still be around and there is always new old stock available from Moto Lita in Britain, you might also check out the British Ebay.
I think all JH's have the same spline connector, held by a single nut. You need an adaptor that goes from the spline to whatever steering wheel pattern you want. It would be referred to as a Jensen Healey adaptor.
When it comes to period wheels, you have a huge selection available. 13, 14, 15" is available, as well as many styles of wrapped leather, wood, and plastic. They generally come in 6 hole and 9 hole patterns. The 9 hole pattern seems to work a little better on the JH in my opinion because it is a larger diameter flange, and therefore the adaptor has more room to make a transition. Most of the JH adaptors I have seen have this 9 hole pattern.
Delta Motorsports in Arizona might still have the original Moto Lita made adaptor, I know because I bought one about a year ago from him.
The horn is a bit of a PITA. I just put a button on the dash. Le Carra and Moto Lita (as well as what appear to be a bunch of knock offs) sell either straight up solid covers or covers set up as horn buttons. The problem is wiring the button, which I never had the patience to attempt. You might need some kind of coiled and expanding wire and a plan to put it into the stock harness.