Usually there is no agency required, you have to get a plan when you plan to visit which country, then you can start to query the embassy`s about how you can optain the visas. Then you can very that informations with other travellers on route - to get an idea.
Depending on your travel speed and visa running time you can optain some visa in front of the trip, others on route (on arrival), and somethimes you had to send your passport home. If you are unlucky you even have to fly home for the visa. Probably you even had to pass an interview for a country like we had to.
It sounds for more work as it really is. Do a approximate time plan for the trip, choose the countrys you like to visit, timeframes per country , check with the embassy (in your homecountry), talk to recent travellers. You can do that in 8 working hours.
Now you can adjust your plans. Check where you had to optain the visa`s in your homecountry, check how long the visas are valid - probably it makes sence to optain them before you start your journeys - and to be there before the visa`s become invalid.
There are some countrys you had to cross, others you can cross - there is no one who "know" the current sitiation for any possible country on the west.
Enjoy the planning stage - it is a part of the trip!
