Hey Engineer Bob, welcome aboard!
I’ve had pretty good luck with Amazon in the past but that’s also where I’ve dealt with a couple of flaky vendors. For bulk purchases like track and common rolling stock, eBay is pretty hard to beat. It helps to look not only for Plarail but Trackmaster as well, since EARLY Thomas Trackmaster items are directly compatible with Plarail (not so much current Trackmaster 2, where the compatibility is kinda sketchy). I’ve also dealt with a number of Japanese retailers over the years with very positive results. The two best known are probably HobbyLink Japan and Hobby Search but the caveat there is that shipping can add up fast, especially on larger sets.
When you finally fall deeper down the rabbit hole, there’s Yahoo! Japan. Since it’s basically a Japanese only auction site, you need to join a reputable shopping service who will receive the items and ship them to you for a fee, of course. That can get quite expensive and is probably way past what you’re looking for as a complete newb. I’ve generally avoided that route so far, so other members here can chime in with better info.
