If this is a PTO coupling, 6 spline shaft, then you should be able to get one from a farm equipment supply.  But if memory serves me right I though the large size was 1 3/8 not 1 1/2.    

I used to work in a farm implement store through college and we had products by Weasler.  They made a variety of different couplers and sleeves.  Check also TISCO, Granger and maybe McMaster-Carr.  Other than that if you have access to driveline tubing you could make one yourself.

Stephan

Hub City makes a variety of shaft adapters

G&G Manufacturing Company out of Omaha, Nebraska makes a lot of shaft adapters and couplers..402-453-9595

What you are needing is a machining problem and not really an engineering problem.

1. Find a good machine shop.
2. Ask them to take a shaft of 1.500 T&G (Turned and ground usually to +0.0000-0.0005 specs) of whatever length you need.
3. Bore out the shaft to fit (most probably to 1.1875 +0.003/+0.0015)
4. Take a 3/8" wide key of the required length. The actual depth of the key may be different than standard and will depend upon choices made in step six. You will need to machine some form of step key.
5. Make sure the key is still straight as the machining will make the material want to bow because of stress imbalances.
6. Broach the collar to accept the 3/8" key. You may wish to mill out the slot. Either method will have its challenges.
7. Install.

This should be doable in about 3 - 4 hours of shop time depending upon your requested accuracies.
If you have some length to play with (at least 3 inches I would think). make a stepped stub shaft where the slide over part on the 1.1875 is of a greater dia. than 1.5" and then change diameter to the required (probably over 1.5 inches or with a large radius using a step). Broach as normal.