How to buy car parking as flat are more and parking slots are less

I booked a flat in a apartment in Pune, in whole project there are 90 flats of which 54 parking slots are available, while booking I didn't opt for Car Parking. Consequently in the registration deed it is only mentioned that I own a flat of size 354 sqft. Now i want to buy stilt car parking by paying a cash of 100000/ against which the builder is giving me a money receipt where it is written that they received the money for car parking booking for flat no . Now when I am urging to do a registration of the stilt car parking separately Builder is saying they can't register it instead they will give me a allotment letter of a car parking on their letter head. Note :- (i already argue with builder that how can you build more flat with less parking so he said who told you that we can't build, since this project in Grampanchyat area no government department cares about it) some research i found below as per supreme court judgement builder has no powers to sell car parking slots allotment letter under his letter head will have no legal value My Question is how to get car parking as flat are more and parking slots are less