hi luke
there is no specifi template made for ijoomla ad agency.
i’ve got the same product as you. the zones are simple position areas in the template, and you can create them yourself if you run out of them – or you can move the positions around if you know your php.
it also depends on what you want your site to look like, where you want your ads etc – whether you want the branding of your site to be the same as the site that will serve the ads etc.
For me, because of full customisation of my template, i stuck with Corona – but that suits me because of the modifications i’ve made and the brand feel it gives.
any template here should do you good. remember to be a little creative so your ad agency gives out that professional look and feel.