Remember that betta might look lonely but in reality most even prefer to keep it that way! If you add shrimps one of three things will happen... your betta will make a tasty snack out of them (most likely), your betta and shrimps will ignore each other (hopefully), or your shrimps might even bug your betta a little bit and nip at him depending on what type of shrimp you get.
Having said that, if you would like to add shrimps because you want to see more than just a betta in the tank. I agree that ghost shrimps are a great start! They are cute little things. They are cheap, peaceful, and being transparent gives them a better chance of not being eaten by your betta. Most of them will die, simply because they are feeder shrimps and usually kept in poor conditions, but the ones that survive you'll know are pretty hardy. Also, be aware that there are ghost shrimps and another transparent shrimp (forgot the name...) that is sometimes mistakenly sold as a ghost shrimp but then turns out to grow big and aggressive and might not be too afraid to pick on your betta. As with fishes, you gotta acclimate them as well. There are a couple different ways of doing it.