Encrypting data using AES 256

I am new to this whole encryption thing. I am creating a program to send some information to a vendor of ours. The problem I am running into is that they require that no padding be applied to the input. From my understanding this cant be achieved using CBC (unless the data is always a multiple of the block size - 256)? Does this mean I need to use a different cipher mode to be able to do this with .NET or how should I go about this? It needs to be encrypted with AES 256 (both key and block size) with no IV.

What are my options to be able to do this?

                    string[] keyArray = key.Split(','); //key is string 32 numbers 45,-12,...
                    sbyte[] sBytes = new sbyte[keyArray.Length];
                    for (int i = 0; i < keyArray.Length; i++)
                    {
                        sBytes[i] = Convert.ToSByte(keyArray[i]);
                    }
                    byte[] bytes = (byte[])(Array)sBytes;

                    RijndaelManaged tdes = new RijndaelManaged();
                    tdes.Key = bytes; 
                    tdes.BlockSize = 256;
                    tdes.KeySize = 256;
                    tdes.Mode = CipherMode.CBC;
                    tdes.Padding = PaddingMode.None;
                    ICryptoTransform crpyt = tdes.CreateEncryptor();
                    byte[] userData = Encoding.UTF8.GetBytes(userId);
                    byte[] dateData = Encoding.UTF8.GetBytes(currentTime);
                    byte[] userCipher = crpyt.TransformFinalBlock(userData, 0, userData.Length);
                    byte[] dateCipher = crpyt.TransformFinalBlock(dateData, 0, dateData.Length);

                    string encryptedUser = Convert.ToBase64String(userCipher);
                    string encrpytedDate = Convert.ToBase64String(dateCipher);

                    string url = baseURL
 + "username="+ encryptedUser + "&timestamp="+ encrpytedDate;

                    Response.Redirect(url);