Foster Smith has some great selections that are pretty cheap! http://www.drsfostersmith.com/produc...fm?c=3578+6635
If you want silk/plastic.
About plants vs silk. How big is your tank? If it is less then 20 gallons go for fake. Plants take a lot of work. first you need to establish them. It is not as easy as it seems. You have to make sure the roots stay in the substrate and they can get loose and start floating if they ar enot anchored/established the correct way.
When a plant begins to die, even a single leaf, it must be cut immediately. 1) The nitrate it gives off will harm the fish. 2)It will kill the rest of the plant(not 100% of the time but there is a good chance).
Now dont be scared if you still want to do it. Real plants are VERY beneficial to a tank. First off they will feed off of the fish droppings and thus the droppings will be a lot less harmful to the betta. The plants also oxygenate the water which is beneficial to the betta also.
Basically it comes down to if you want and are willing to do the work. It will mean you will have to put in a bit of extra time with your betta for the benefits of live plants
If not buy some off of the site I gave you, I have been using them for years and they look amazing!