We used a pile of the ladybird books, which DD loved, got them second hand from ebay! Plus I trawl the charity shops for books!
Also in case you want to use the oxford reading tree books...
Oxford Owl
...offers a selection of their books, which anyone can access for free!
We found the library didn't offer a lot at the lower stages when DD first started reading but now only a couple of years on they seem to offer a bigger selection, may not be true for all libraries but certainly worth a look, especially as books are expensive when they often only want to read the really early books once or twice. Ones to looks out for that we found helpful: series with a turquoise tadpole on the spine; or series with bars on the spine in pink, yellow or blue and 1,2 or 3 bars depending on difficulty, which also comes as a phonic version; or series with a childs head poking over the top of an open book (book cover in host of colours for difficulty level).
Each reading scheme teaches words in a different order, have difference sentence structures etc so it's best to read as many different schemes as possible so that they don't become a reader only able to cope if you give them one scheme! Plus there is a series of easy non-fiction books on bodies, food types (and something else) which DD enjoyed.
Plus if you hunt through 'picture books' you will notice that they come in a range of levels, so your DS could have a goo at some of those! I used to encourage DD to read the words she recognised as I read a story to her, let her practice some things without feeling upset by not knowing all the words!!