Campbell had a network of informers who transcribed accurately, oral tales told in gaelic by highland Scots. The book contains tales (sometimes more than one version as is the case with the Sea Maiden) in gaelic and an English translation. Here are a few examples (not comprehensive) of where it can be sourced, I have tried to link directly to the relevant web page on the site where possible. If you are wanting the Sea Maiden make sure you get volume 1.
Online Books-Full electronic copies of facsimiles of editions West Highland Tales can be found for free on the internet. For example The Internet Archive and Google Books
Online Booksellers-for example Amazon , Abe books
Book shops-(you may have to order), you could try finding an independent local bookshop in your area