There are several SaleHoo listed suppliers for baby onesies and rompers, however their products are made of the cotton material. For the merino wool baby clothing, I did a search outside the directory and found this one you can use -
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The website has been existing since 2003, and to apply as their reseller, go to Link hidden: Login to view
Please note that this was found off SaleHoo so we advise that you use safe payment methods.
Hope this helps. :)