I tried a couple of bought ones and they didn't really work as incentives. But a sheet of dinosaur or postman pat stickers for either of my ds's to take one and stick it on a chart he's made himself... and they are the most obedient motivated children in town.
ELC have magnetic ones which we have for both DD (5) and DS (2). They are good because you can reuse the magnets at the end of week and write on chart what behaviours you will reward and what rewards will be - then wipe off and change as appropriate. Have used DD's for last 3 years - and it has developed with her. I think they were about £6 - which i know is more than making your own but would say is worth it.
Star charts are all very well, but DD (3.7) got bored of star stickers. These days we go and buy a packet of whatever stickers she likes at the time (fish / fairies / dinosaurs / butterflies etc) then I draw an appropriate background - the picture doesn't have to be great, my artistic ability is fairly limited! A couple of flowers and some grass was about it for the butterfly chart!). She loves that and is very motivated to fill up the picture, whatever it may be, so I find that works best for us.