"Try the thrift store" is the perennial answer which in most cases for Doctor Who will not work. However, for this outfit, you should be able to get a lot of it at Goodwill. :thumbsup
The easiest way would be to get the shirt and jacket from a thrift store, a pair of army surplus jump boots, and a pair of skinny black trousers from H&M. Get your suspenders and bow-tie online, you're not likely to find a matching pair anywhere else. You should probably be able to do it for around $75 total.