We got the same letter from US Cellular.   I went into the local US Cellular office and they said no flip phones.   The day I went to Verizon to switch carriers ,  I didn't know about needing my PIN # and had to go to US Cellular, down the block,  to get it.   THIS US CELLULAR regional office informed me they still had flip phones.  And they would continue to have them.   I was pissed about being lied to and said no.    So my first suggestion is go to a bigger US CELLULAR office and see if they can buy newer flip phones.    My husband insisted on a flip phone and I switched our plan to Verizon.   He got a brand new flip phone just a newer version of the phone he had.   So far,  at 1 month,  he is happy.      
Plus,  we were told the old phone would quit working on Jan. 1st.   Better check on that.