Hi,
There is a shop which sells and hires mountain bikes in Mojacar. The shop is situated on the same parade of shops as MacAnthony Realtors and the "Subway" sandwich take-away.
If you can't find what you are looking for there, then Carrefour at Almeria have an assortment of bikes as do all other Carrefour's.
Hope this helps, but if you are really stuck, there are two bikes chained up where the cars park, and I've got a set of bolt cutters!!!!!!...................only joking Ken!!!!
Regards,
Lorraine Braid.