I've given footballs as boy's party bag presents - I got the 99p ones from Asda. I had one giant party bin bag that held all the footballs, made the kids do lucky dip to pick one each. The party bag (provided by the venue) just had a slice of cake in it.
Worked out really easily - I knew exactly how much I was spending on party bag filling (although I had about 20 rather than 40!), shoved them all in a big bag and that was it, no hassle doing fiddly filling with lots of bits and bobs. then just did the cake at the party after they had done blowing out candles and singing happy birthday.
If you do go down that route, it's worth ringing up asda and seeing if they will put the number of balls you want by for you - they don't always have 40 in stock, plus if they know you want a lot like that they will be able to put by a single box which will make it even easier rather than having to have them all loose in your trolley! I looked in lots of places and they were the cheapest place to buy the balls (albeit only a penny cheaper than the pound shop, where they also had silly face balls, not just footie ones if you want to have some that are less overtly boy-ish if you will have lots of girls).
The next year we put in watches from sports direct - which were 50p each
www.sportsdirect.com/multi-watches-899023
and a 35p tennis ball from there too, and a small bag of haribo. plus a slice of cake. All came to less than a pound each and went down well with the boys (it was for a 7th birthday) who were much more impressed with the watches as it was something different and they looked like they were much more expensive than they were 
. Advantage was they were small and easy to carry - especially when compared with the footballs of the previous year!
It's worth browsing through the sports direct sale section and accessories sections as they have some stuff that is really cheap if you want other party bag filler stuff. Other friends have bought good stuff from ebay, including sets of spy pens (one of invisible ink, one that shows it up) that I remember from my childhood, wash on/wash off tattoos and colouring sets, all for less than a pound each.
And if you are looking for books - look at thebookpeople.co.uk as mentioned, but also look at bananas.co.uk - they are the clearance site for bookpeople, so have stuff that has been marked down even further. But they don't have loads of stock for each thing as they do on the main site (obviously!) so if you see a set of books that is right then it is worth getting them there and then. Stock seems to come and go in waves - sometimes you look on the site and realise that there is lots of stuff that is out of stock, other times they still have all the stuff listed on it. I haven't stalked it for long enough to figure out if they have a pattern and say put a fresh load of books on there every friday or the third tuesday of the month or whatever, so that if you look at it just before they put more stock on then it doesn't have much stuff or if it really is just luck of the draw!
Whatever you end up doing - hope your ds has a lovely party and I'm sure the kids will all love their party bags, whatever you put in them (well, give or take a few carrot sticks 
)