Probably 90% of my plants I get off eBay or online sellers cos the garden center / nursery plants here are usually way too huge, or will be before long. Plus who knows what they've been sprayed with along the way to your vivarium.
I also buy terrarium plant "unrooted pruning grab bags" from people off the various online forums I frequent. Many forums have a classified section that can be quite fruitful to explore when looking to expand your plant list but being as they're hobbyists like us you have to hit them at the right time cos they don't usually have more than one or two "grab bags" of cuttings at any one time but you can wind up with all sorts of neat things (some named some just with a "?" on the baggie).
I hate shipping fees like anyone else but if $5 - 10 in shipping charges is the difference between never having certain plants by next week or never having them then the choice has already been made for me! LOL