Definitely buy from members on here. LFS and big chain stores like Petco and Petsmart are expensive in comparison and have "common" plants, while members on here have much more variety and great deals at that.
Only thing to watch out for is if your set up is up to par (no pun intended haha) with some of the more demanding plants (some require high light, co2, additional ferts, etc). But still there are plenty of low demanding, beginner friendly plants here.
There are other sites (HANaquatics, bamaplants, the one smackey listed above) that are nice to order from, but still the members here usually have better deals. The sites might have more species available so you can order from one source though. But I've saved a ton of money getting plants through members here so I highly recommend here.
Mention your tank dimensions, lighting, substrate. Planning on fertilizers or co2? How do you envision the tank to look?