What you buy depends on how comfortable you are with using and storing dangerous chemicals, traditional pastes are a mix of hydrofluric and nitric acids in a gel base, some useful info and links in this recent thread "Clicky"
There are friendlier ones based on citric acid,
I use Antox, or pelinox, these are traditional pastes and work very quickly, citric based pastes tend to take longer to work, at least the ones I tried did. Also there is the option of electrolytic cleaning, but I find it less efficient in difficult corners,
Welcome to the forum