Back to the original question: "Does anyone have a source for old Siemens ET200L Profibus Modules? ".
I tried ebay.de (a good way to know if it is possible to find cheap Siemens parts) and found nothing for ET200L or ET200 SC.
Then there are the professional resellers, where you may be lucky to find a part here and another there, but not cheap. They know how to charge !
And of course, you can contact Siemens on a per part exchange programme. One defective part returned grants you the right to buy another refurbished part - VERY expensively.
Oh, and you have used counter modules. A bit out of the absolutely trivial mainstream. Then it will be even worse.
Nah. You and your customer just have to face the reality.
And consider yourself lucky. It will be easy, and ET200S is quite cheap really.