I live in Istanbul, I love espresso, but most espresso here is 100% Arabica. Robusta coffee is hard to find. What alternative coffee do you recommend to get the right caffeine during the day?
A possible different approach. Getting the perfect flavor is hard enough. The more constraints and tangential variables you introduce, the harder it is to produce a desired flavor profile. You say most espresso in Istanbul is made from 100% Arabica, and you love it. Your objective is just to get more caffeine. My suggestion would be to not mess with the flavor you love as the mechanism to get more caffeine.
Robusta has more caffeine, but it also has a different flavor. There are also some Arabica varieties with more caffeine. But caffeine is very bitter, and if you substantially increase its content in the brew, the result will be noticeably more bitter. So you may sacrifice the flavor you love to get more caffeine from that source.
An alternate approach would be to just take a caffeine tablet when you want the pick-me-up. You can even wash it down with coffee that still tastes good. :-)
You could try to find Vietnamese coffee in Turkey. Many kinds have high caffeine content. If you find them, you could get a phin for the preparation. Then these coffees will test best.
How about Taft coffee? They are known for their high caffeine levels. You can order online.
Just checked their website here, they claim to have 23.2 g caffeine per 1 kg of coffee. Actually I also buy Taft Coffee. I am not really considerate about caffeine levels yet it tastes really good.