TD sells a broad range of mutual funds from many fund companies. In the buy & sell window you need to select the mutual fund tab. Also they don't sell F-series funds through TDDI because those are intended for investors that use fee-based advisors. And if a D-series fund is available the platform should redirect you to it from a higher MER fund that is intended to be sold to investors using advisors. There used to be a list of mutual fund companies that TDDI sold on their website, but I cannot find it now.
Is there a particular fund you are looking for?
EDIT: Meritage looks like National Bank's in-house funds, so unlikely they would be available anywhere other than National Bank. Other in-house funds like Tangerine, Investors or Quadrus would not be available at TDDI.