should i buy a ukulele capo?

If you are playing a Ukulele as an accompaniment instrument to your voice, I strongly suggest that you purchase a Capo. Capos are specific to the instrument used and while a guitar capo might work on a Uke, I would still suggest a capo made for the uke.
Quick tip; always keep the capo in your accessory pocket of your gig bag or case so you always have it with you.  

Back in my days as a novice player, I would simply learn songs in the keys that the recordings were in, and if that key wasn’t in my vocal range, which often was the case, I just resigned that I could not sing that song.  Undoubtedly, you also have learned the chords of a favorite song only to find your voice does not match the key in which you learned the song.

At this point your choices are;  keep practicing until you can sing the song (the hard way) , learn the song over again in a different key (the long way) , or simply use a capo to change the key (no re-tuning required) to match your voice while utilizing the chords you already know (the easy way).

For Example: The standard tuning for a ukulele (GCEA) is also called “C”-tuning. Placing a capo up on the fourth fret will change your ukulele’s tuning to an “E”-tuning and you will play the same chords you already learned. You should practice moving the capo around until you find” ‘Your Key”

Remember there will always be a key that will match your voice, enjoy the journey.