My first go-to: For Sale section here on the forum. Then the Want To Buy section if I think someone may have what I'm looking for but hasn't posted it.
Then I hit up tried and true sites like Green Leaf Aquariums and Aqua Forest Aquarium. They both always have a ton of tissue culture varieties and are good at shipping in hot and cold temperatures.
Glass Aqua is also a source for tissue cultures.
I don't want to get into vendor reviews but Aquarium Plants dot com also has some good options on occasion.
Once you get into the hobby a bit more, you'll find that hobbyists almost always have the best stuff.
Also have to second what
@Griznatch suggested. I've routinely found great tissue culture options at Petco and PetSmart. Even the occasional tubed Anubias or Fern that's in good shape. It's hit-or-miss but always worth a look or two to see what's available if you're already there.