Ive never had a prob with any bucket and ive used em from specialty hardware stores, hydro shops, HD, Lowes and Wally world. Just paint em black to keep out light and your good to go.
plastic 5 gallon buckets are just thick poly. Think about them as really thick garbage bag, pond liner, or poly sheeting. my point is it is inert. if you search around you can find black ones, or you can you paint it or tape it.
number five plastic is what you want. check the number within the recycle stamp printed on the bottom of the bucket.
10-gallon black gro bags work just a well too! You can roll them down to any size you want. Start at 5 gallons, and as the plant grows remove the lower branches and increase the volume of soil until you've reached the bags limit to create a 10 gallon size root mass.