you will find leather affordable ones in a tack shop in sharjah equestrian club its called Al Maydan , alternatively i saw few in horse world in falcon center in nad al shiba
You can get horse riding items, including boots, from Decathlon in Mirdif City Centre. They have a decent selection so you can go cheap or expensive. I got the relatively cheap ones because I was trying to figure out if I wanted to pursue riding or not and they were around 90 dirhams.