i am about  build a propane froge first one . my question is can i use the regulator off of a acetaline bottle set up or do i have to have the adustable one from a propane  supply house i. i can get a regulator from harbor frieght for 35  and a hose for 15  

If the acetylene regulator is rated "for all fuel gasses"  then yes; if not then no.  If not marked then no.

Depending on the size of the forge you may be able to get a propane regulator at a hard ware store depending on what part of the country you live in.  The adjustable regulators used to boil or turkey fry are often used.   Personally I recommend a propane regulator from an Industrial Gas supplier which has a higher volume delivery, better control, and gauges. There are some things I won't buy from harbor freight.  Gas welding/cutting  equipment is one of them.   Too many safety issues connected with their suppliers.

Excellent summation, Charlotte:

To not using Harbor Freight regulators, I would add, not to use Mr Heater regulators, and cheap no brand name regulators. the price difference between the worst and best regulators is between $10 and $20, depending on size; talk about false economy! Also, I recommend regulators designed for LPG over standard torch regulators, because LPG regulators are more resistant to "freezing up" under wet winter conditions.

You might want to consider one from a company that sells forge making supply's. I am going to be making a gas forge and although it seems expensive, I am going to get a single burner regulator kit from Zoeller forge. Well that is if you are using a single burner. If you need one for two or three or how ever many burners, they sell them too. 

They sell a good regulator at a reasonable price; Fisher brand, if I remember correctly.