A ukulele tuner is a useful bit of kit to have. But they're by no means essential. A cheaper option for making sure you stay in tune is a set of ukulele pitchpipes.
A ukulele pitch pipe has four holes. Each one representing one string on the ukulele. You blow into it and tune your ukulele to the note.
There are a few drawbacks with pitch pipes. Firstly, you have to buy one for each tuning you use. So if you play in C and D tuning you'll need two sets. Also, if you use an unusual tuning, you might not be able to find a pitch pipe matching it. Using pitch pipes can also be quite tricky if you're playing somewhere noisy.