Yes do look on ebay. Set it to search by nearest to and then see if there are any to pick up - postage really bumps the cost up. Also, if you search for "Happy street" you can find the older stuff which comes with bases for the shops/houses and a road system. Stuff labelled as "Happy street" tends to sell for cheaper as well, because not as many people search for it. Sometimes the road system/bases come up under "Happyland road" searches as well.
DS started off with the emergency vehicle set at 14 months, which he just used for ages as toy cars and liked the sounds, then someone bought him the plane, which he didn't really play with until he was older, and my mum bought him the big wheel (from fairground set). These were all unrelated, but he loved the way he could put the characters from the different sets in the cars etc and we had fun making people fly off the ferris wheel and need an ambulance . I noticed they had some of the shops at his childminder's, so I asked her if he played with them much, and she said that he did, so I bought a job lot from ebay, which was a police station, petrol station, some of the road and a construction vehicle/worker/prop set.
I recommend getting the ones which fit together with many of the others, rather than the more random ones like the pirate ship, dinosaur, fairy sets etc (though I suppose you could have a fairy village with shops etc, or the pirate/dinosaur pillaging the town!) just because I think it works nicely as a town, you can have as little or as much as you like, and it's a good thing to tell relatives about for christmas presents etc, and dinosaurs/pirates etc, there are always standalone sets of these which might be cheaper or more detailed or something.