This is actually something i found quite challenging when I first moved here I must admit.
I always found it bizarre when I would visit a country such as Sri Lanka, and ask someone a question in regards to purchasing something or getting some kind of service organised (tuk tuk driver, carpenter, laborer for example), and no matter what I needed or who I asked, the person would ALWAYS know someone who could take care of it. Whether it was a family member, a friend, a friend of a friend or who knows who else, the fact is they always knew someone who could handle the job!
It wasn't until I moved here that I realized how much making connections with things like that was just a part of the life style.
Back in Australia it was easy to just jump on google and look up the closest plumber or electrician and give them a call and get them to come and do the job I needed. In Sri Lanka however, I hate to tell you but your old pal google isn't going to be as helpful as he once was back in your home country. You can't just google the closest timber shop, car dealer, restaurant and have a million options pop right up to ring and compare prices between. It all comes down to word of mouth and knowing the right person here.
It is going to be tough when you first get here i'm not going to lie, not based on the fact that you are limited on who to ask (seriously, there will be random locals walking up to you on the street asking you where you are going and telling you to follow them to their friends shop), but finding someone TRUSTWORTHY to ask, now that's a different story....
One thing I can advise you to do would be to join some of the Sri Lankan Facebook groups in your area for expats and foreigners. The people in these from my experience have been very helpful to newbies with any questions and advise they can give. Remember, everyone was in the same position at one point! You can post what you are looking for on here and in what area you are looking for it, and more often than not you will get a quick response from someone who can give you a name to go by with their personal recommendation. I have used these groups many times with great success.
Trail And Error
This one is going to seem pretty obvious, but it's something that is going to go hand in hand with moving to a new area. That being, just trying new places in your area and seeing what you think of the service they offer.
I hate to say this, but at some stage you probably will get ripped off a little here and there no matter how cautious you are, its just a part of being in a new country i'm afraid. But until you have someone else you trust's advice to go by, I think it's best to literally just walk around and check out the area of where you are now living and see what it has to offer. Speak to the locals, the shop owners (if you can) and see what advice they can give. They are always very friendly and willing to help foreigners. Go with your gut instinct and trust your judgement on the people you speak to. You can generally tell straight up if someone is genuinely trying to help you, or they just see an inexperienced foreigner and an opportunity to take advantage. Hopefully you will be able to tell the difference between the two, but if not you will learn fast!
It will take you a little while to adjust if you are from a country where business is very much online based and you can just search online for whatever you need, but you will get the hang of it, plus making connections is a good way to become apart of the community and meet some lovely people in the process!