Should you take up training only in the location where you are at? The answer to the questions is both a yes and a no. Such answers are often very irritating, but in this case, they are sadly also the only answer one can give.
If you are for instance in tirupati, and what to take up GRE coaching, there are barely any GRE coaching centres in Tirupati. While you might find some places that do offer training, you will realise that the quality is quite often suspect. The problem is natural, there are not many trained faculty or even most experienced faculty in those areas. The reason in simple, whether you are looking for the best IELTS coaching in nellore, or best IELTS coaching in vizag, you will not find many, because most coaching centres are concentrated in cities where a large number of test takers come to take these tests.
So, if you want to take coaching in one of the best coaching centres, your only options might be to come all the way to the nearest education hub, or to take some really good online coaching for PTE, IELTS, GRE, or something else. The best online coaching usually offer interactive sessions where the trainer is exclusively present for the online session takers.
If you are looking at more niche areas like coaching for OET, then the chances are going to be even more slim. Not only do many institutes not offer online coaching for OET, they do not even offer classroom coaching. OET coaching is generally more expensive than most others because of the specialised skill set that the trainers need.
If you do not have access to coaching or the internet, your only bet would be to look for the best OET material on sites like amazon.
Good luck for preparation!