the only registers of births are those under civil registration (voluntary from 1849, compulsory from 1878) - the indices to these registers are available on Mormon microfilm (see the explanation on Brain Lawson's site) but the films are not for sale that I am aware of - certainly those post 1911 are available only via General Registry.
Prior to this the registers were of Baptisms (not births ! though unless you are mormon baptisms are generally performed on the living and within a few weeks of birth) - these are the parish registers, the 1910/1911 official transcription which forms the basis of the IGI entries - the copyright in these films is held by Manx National Heritage and I believe copies can be bought if various copyright declarations are agreed to etc. Some of the actual parish registers are also now on film (some early copies via Mormons - later ones done by MNH as the registers are transferred into their keeping) - it may be possible to acquire copies of these as the IoMFHS have bought a number of microfilms to augment their library in Peel.