You can get a kritter keeper off Amazon, Petsmart (in the reptile section), or Petco.
For a filter, you could either get the Tetra 2-10i power filter, I have one and it works great so far, but one that works REALLY well that I have in 2 of my tanks is a DIY filter. I just used a small 12 fl oz coke bottle with a airpump, air stone and tube (good price too):
I don't use a pipe, it works better with one but it works noticably well without one too. I tried it in a 3 gallon tank (an air pump for 10 gallons) and poured a ton of flakes in the water, that thing sucked it up like crazy!
And since it's smaller you can hide it behind plants.
At the same stores I mentioned above, you can find a variety of foods for bettas, like freeze dried bloodworms, shrimp, etc. Hikari is a good brand, so is Omega One. As long as you don't overfeed on the freeze dried ones and you pick a good brand, then it shouldn't cause constipation.
For water conditioner, Prime is good as it conditions the water and detoxifies Ammonia, Nitrites, and Nitrates, but I use Tetra AquaSafe water conditioner and it works great. Prime is a good thing to have on hand. You can get big bottles of those and they do last a long time, only using a 1 or a few drops depending on the size of tank. For a 3 gallon, it's not alot.
API water test kit! It's a bit expensive, but really worth it as it lasts a very long time and is very accurate.
API Aquarium salt is great, but only use a very small amount for up to 10 days as a treatment.
As far as silk plants and heaters, I can't say because some people know better ones than I do that are adjustable and turn on/off and I don't use silk or plastic plants anymore.....well, not right now. Live plants are awesome and can be easy to care for if you pick the right ones. But that's just my opinion!
Do get a thermometer to check the temperature!
Hope I helped,