I am in the same situation as you and simply bought the ikea ones. They are very good and cheap.
Just bought two two days ago. Easy to transport as comes in a Compact form. You will need to open it one day before they arrive for it to take it's normal form. ( not sure what I m saying makes sense lol)
I have a double blow up mattress that you can borrow too. If you wanted to take Izzy up on her offer and take mine too you could split the kids up, if you have the floor space for two double mattresses.
I also have a single foldable mattress, that you can have. Its not overly thick or wide but it might do for a child. It is one of these patterned mattresses that you see in Carrefour/Geant/LuLu.
There is a shop on Sheikh Rashid road (the one with lamcy on it) that's pretty cheap - can't remember the name but neither the junction with Sheikh Khalifa road (the one with Burjuman on it)
Leaving it till the last minute as usual :D. My sister and her tribe arrive on Boxing Day, by which time all the beds in the house will be full bar the bed reserved for her and her DH. So I'll need two single mattresses for my niece and nephew. Any suggestions? I really don't want to fork out a lot as they'll only be in use for a couple of weeks.