Clark rubber mattress?

Hi All!

I'm really excited, I found my dream cot second hand. Its the Ideal Arms Reach Co Sleeper. It's a third of the price of a brand new one!

Only issue is, obviously I'll need to buy a new mattress, but it is a reallllly weird size. I cant find anything online. The dimensions are 99cm x 59cm x 3cm.. and oddly they don't sell them on the brands website.

I've heard a few people recommend having a foam one cut at Clark rubber.. are these SIDS approved and safe for a newborn?