This is a very good question that I get from my students all the time. Improving spoken English without a speaking partner is very difficult. Even though
there are many ways of doing it as others have mentioned.
You need to understand here that you need to use English to improve it and to maintain your proficiency. It's not practical when you don't have anyone to
speak in English with. You can't possible keep studying it forever. Many people think that if my English gets better once then I will create an environment
of people around me who speak in English. When students start learning English with that mindset, in most of the cases they give up after a few days before
reaching their goal and then as they have none to practice what they have learnt they again go back to square one.
It's also not recommended by me to rely just on a speaking parter or partners to improve in English. You need to balance it out. You can do your own self
study and at the same time try to meet and speak with people in English as much as possible.
So, instead of finding out on how to improve spoken English without a partner. I think it will help you more if you can do some research on how to find
people to speak in English with. In fact, a lot of people join English classes for the same purpose so that they can find other English learners to speak in
English with. These days there are online and offline courses which you can join.