Are there tours to see Western Lowland Gorillas?

I believe you can visit them throughout Nigeria, Central African Republic and Cameroon. I don’t know how it works there and the costs for the permits, but I am sure it’s cheaper than mountain gorillas. I visited the mountain gorillas in Uganda a couple years ago and it was the best experience ever. I can answer any questions you have about mountain gorillas, since I did a lot of research before my trip. Rwanda is the most expensive place followed by Uganda which permits are almost half the price than Rwanda’s, but still really expensive. Another option you have is the DRC (Democratic Republic of Congo) permits are about $200 usd which is way more accessible than the other two places, but its political instability and its armed conflicts make it a risky trip, I know people who have done it though. For any of these options I will recommend you to do it with a guide that can drive you to the national parks without any problem. You can book them with locals that will charge you less than the big international companies.

DISCLAIMER: all of the information below was found by Googling. I do not have any personal experience with gorilla trekking (more's the pity.)

It is possible to do gorilla trekking in Gabon, at Loango National Park. However, there is only one group of Western Lowland Gorillas in that park that has been habituated to human contact:

Despite gorillas being the largest primate species, it is difficult to see them in forest because they are naturally afraid of humans and typically will flee or aggressively charge if people get too close to them. Gorillas that are visited by people have undergone ‘habituation’. This refers to the process, where through daily peaceful contact with humans, gorillas have slowly lost their strong fear of humans and have learned to view them as neutral beings in their environment. The Atananga Group has been habituated since 2014, with tourism beginning in 2016.

Access to this group is regulated; only a small group of people can visit the gorillas for a limited amount of time, and only a limited number of times per week. I was unable to find costs for these tours online, but googling "Gabon gorilla trekking" yields several operators advertising this experience.

Two other groups of habituated Western Lowland Gorillas also exist in the Dzangha-Sangha area of the Central African Republic. The above link also claims that

There are only three habituated groups of western lowland gorillas in the world, and two of them are in Dzangha-Sangha.

The third group is presumably the Atananga Group in Gabon.

In contrast, according to this tour operator's website, there are many groups of mountain gorillas in Rwanda, Uganda, and the DRC that are habituated to human contact. Moreover, mountain gorillas tend to stay relatively close to each other in large groups & stay on the ground, while western lowland gorillas often climb trees and spread out from their neighbors. This makes the mountain gorillas more appealing to the causal tourist, as you are more likely to have a good view of a larger group.