The big problem with making a "stay alive" for TT is the capacitors needed. You need capacitors high in value, but small enough in size to fit in a TT:120 loco body. As has been said such items are very expensive. It is possible, but you will need a mortgage to buy the end product. The design of a "stay alive" is simple enough, "stay alives" are nothing special, usually just made from a few common electronic components. Whilst most modellers think nothing of spending £200 plus on a loco, when it comes to other kit most modellers have deep pockets and short arms unless it's dirt cheap, cheap, cheap. It wouldn't be commercially viable.