The realtor I planned on using just told me that since I had been in contact with the builders sales agent they( builder) would not let me work with a realtor?? What !
Yes, this is common. The builders will only pay the Realtor if the Realtor goes with you the first time and 'registers' you as their client.
If you have not signed any docs, try simply telling them you will not sign a purchase contract unless you are able to be represented by your realtor. Many times we give up rights as consumers. The fact is you do not have to buy their home any more than they have to allow your realtor in on the deal. I would be very interested to see if they let a deal walk because of a decision that won't cost them anything to change their mind.