The only way at the moment , unless there is another crowdfunding round is to buy them from a seller that bought in previous crowdfunding rounds through crowd cube , the seller can only sell shares in the block that they bought them, they also need a valid reason to sell them which must be agreed with the Monzo board prior to sale , if its been agreed by the Monzo board I think crowdfund deal with the paperwork for transfer ( you also need to be registered with crowd cube I believe )
the wiki for selling is here Monzo Shares for sale Wiki