These are great value, I use one all the time when diving.
About £12 at the dive show, between £15 & £20 from dive shops.
Good item to buy on line as it is small, light and p&p is cheap .
Try kentdiving.com - good value and reliable
Scubafill
PADI Divemaster
Look around & you may find one that's a small torch as well - strobe one end & torch the other. Be careful with the strobe though. They're not too robust. I have at least two dead ones lurking in my dive bag that I'll throw out one day when I remember & I'm near a bin.