you can request a car seat when you call for a taxi - they generally have them located at their base - but you'd have to book well in advance

you can supply your own seat - depending on where you're going and whether you'll be able to leave it somewhere when you get there

all taxis (at least the ones locally - my dad drives!) are fitted with car seat fixtures - they just don't have the car seats in the cars all the time due to the space they take up in the boot, and the risk of them being stolen...