I just replaced my LP alarm. I also wanted to detect CO, so I replaced it with a combination LP/CO alarm. It is made to specifically fit the cut-out of the detector that came with my TM, which was made by a different company (CCI Controls) now out of business.
So if you have an LP detector, model 7770 (which is marked on the back of the unit, visible in the bathroom cabinet) that looks like the attached pic, the unit linked to below is a direct replacement with added CO monitoring functionality. It takes 10 min tops to replace, and you need only a screw driver.
While the LP-only detector is easy to find, I could only find the LP/CO detector at 2 vendors (priced the same), one of which is below. They shipped the same day I ordered, were responsive to phone calls, and shipped a fresh product (according to the date stamp on the back).
They shipped a flier with my unit with a coupon code on it for free shipping for $99+ orders. The code is SEFS99. You'll have to order a $1 item to use it.
http://www.safehomeproducts.com/shp2...79/249779.aspx
Dave