The tomix percy would roughly match for wheelbase, but would require quite a bit of hacking at the underside of the boiler to try and get something to fit, and then it mightn't look right when finished. I designed it around the fleischmann 7000 chassis, so I'd definitely recommend using that if possible. as gerry says, eBay.de is the place to go, just find a seller who will post worldwide and takes paypal. If they don't, often if you send a message and ask, they're cooperative. in germany they tend to cost half what they do in the UK (the UK prices tend to be £35-40), whilst I imagine that over the atlantic they'd be either very expensive through specialist importers, or unavailable. If you can, try to get the version with the metal chassis (the chassis block is black in colour) as opposed to the newer red plastic version. The metal ones have more weight, and the rear of the crosshead is thinner. I discovered when I went to build my third one that the crosshead on the newer plastic chassis is a little too thick (maybe 0.5mm thicker for some reason) on the back and fouls the coupling rod. There are two solutions to this - either don't fit the coupling rod (not really noticeable), or file down the back of the crosshead until it does.
Sadly this slight problem only became apparent 5 months after I'd finished designing the loco, all the other parts of the two chassis versions are identical and interchangeable. I have 8 of them and they're practically bomb proof and run well, hence I've used them for all my designs so far.
I'd recommend this one:
the seller will post worldwide and has kindly included a photo of the bottom so you can see it has the thinner crossheads. Around £30 is the most I'd bid for one, with most of mine coming between £15-£25 including postage to the UK.
Either way, best of luck, there is some information on how I finished mine off on my website: http://brackmodels.weebly.com/index.html
Ironically I don't actually have one at the moment as I sold one and the others are away on loan, but I do have a part finished one I ought to get around to.