Nintendo and Namco have both started threatening legal action against anyone distributing illegal copies of their software, specifically singling-out ROM sites and sales of the xxx-in-1 JAMMA boards. Nintendo seems to have been particularly aggressive with the threats lately. The result is that most of the previously well-regarded resources for ROMs that existed for years abruptly stopped hosting them around early-mid June, and several US-based companies which previously sold the xxx-in-1 JAMMA boards (which are NOT licensed) have stopped doing so over the past few months.

It looks like the only way at present to get ROMs fairly easily without risking the dangers of sketchy sources is through archive.org. (They were granted some type of legal exception which allows them to host the ROMs legally; the catch is you have to download the full sets, which are 50+ GB.)

If you want a JAMMA board, it looks like the easiest option is Hong Kong based sellers via Amazon or Ebay.