Much depends where you are and on budget.... There are a lot of repro places online. Aero's 30s trousers are fantastic; Cathcart London I am also well pleased with.
At the more budget end of the market (and depending on how wide you want....) I've been very pleased with some trousers from Peterchristian.co.uk in a sort of English 50s width with a good waistband on the actual natural waist. For the Summer, I recently picked up three pairs of sofmilitary.co.uk (if you're in the US, Whatpriceglory.com is great) repro WW2 US Army chinos. WW2 era milspec repro is an excellent way to pick up an affordable pair of trousers that with care (avoid thigh map pockets and ankle straps, obvs) can be easily adapted into a civilian look. With cotton of course you can always dye them, as I plan to do with two of my pairs of new chinos.