Conveniently, there are two different types of MAP sensor fitted to the Peugeot 306 GTi-6 depending on age. 1991 - 1998 (phase 1 and 2) are different to 1998 onwards (phase 3). The earlier type is considerably more expensive.
For the earlier type, as found on phase 1 and 2 (up to 1998) the Bosch part number 0 261 230 012 (Peugeot part number 1920 9H). At the time of writing this is £73.48 incVAT from Peugeot and it is normally cheaper to find a local motor factors who deal with Bosch and they should be able to order one in (my local place had one on the shelf which was a nice surprise).