I use keto-diastix which are available online quite cheaply.
Its hard to get them to wee, but if you wake them up (not too early!) and pop them in a plastic tub (no bedding or anything) then quite often they will urinate - one of the things hamsters do when they first wake up is go to the loo!! Or I did find that after exercise pop them in a tub with just a water bottle, they are thirsty so drink then wee