I’d suggest looking at the single button extension shifters with one programmed for shifting up and one for shifting down. This will keep your hand position symmetrical.

If cheap is paramount you can buy single shifters. Universal Cycles is a good option.

You will also need to select a Junction A and depending on what Junction A you pick a Junction B may be needed. Junction A’s come in 2, 3 and 5 port varieties and provide an internal battery charging port and system control. Junction B’s are “dumb” and simply connect multiple wires. You need to connect the battery and derailleur to the shifter(s). Each connection needs a Junction port. If u put battery in seattube or downturn a Junction b might help, but you could also just run extra long wires from the Junction A if ports are available.