Seeing as how the whole point of IDPA is to attempt replication of defensive pistol work, I think that an IWB would be fine. Howver, I have only shot IPSC, due to the fact that we have no IDPA around here. Just MHO
ive shot idpa with my xd40. its classed as esp. thats because batf classed the xd as a single action. if this is your first shoot, you can probably get away with any holster. you will be expected to get an idpa approved holster. the book has a list of holsters availible, but the list continually grows and only updated every 2 years. most holsters that have been qualified idpa will be advertised as idpa legal. its a selling point. im waiting on my g-code, its legal. mark waldon has some that are legal also.at least he did when his site was up before the big move. remember, just because its not on the list doesnt mean it hasnt been approved, check makers site. have fun at the classifier.
Dogpack is correct regarding the Mark Waldon holsters being IDPA approved. We carry the Waldon Texas Sidewinder for the XD-9 and XD-40 4", and we've confirmed with Mark that it is IDPA approved. As far as leather IWB's, it's hard to go wrong with this one. Here's a pic and link for more info:
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