I never liked the feeling of a smaller tube stretched to fill a bigger tire but it does work. Alternatively you might get away with using a fresh 26x3 tube as they are often undersized.
You could go down to your local bike shop and see what they could get for you, or someone else here might know of another online retailer to look at.
The big difference is if you use an undersized tube at low pressure most of the pressure is being exerted on the tube just stretching to fill the tire, where a big tube will not have to stretch so much and the pressure will be more directly pushing on the tire itself.
Eric aus Chemnitz made a good suggestion. I have not used the Schwalbe tubes but I have heard good things.
I’ve run a 26x2.0-2.5 in my 24x3 for quite a while with no regrets. I don’t think I’m running as low pressure as some people so that could be a factor. I usually go with 2 bars, or a shade under 30psi.