A general good quality "pine" to ask for is "unsorted redwood" you will have to go to a timber merchants for this not a DIY shed or builders merchants.
If you want a bit more grain then something like "southern yellow pine" looks nice, supply as above.
Both of the above are sold with a sawn finish, typical sections for the redwood are 225x25 and 225x50. If you don't have a planer/thicknesser then some of the timber merchants keep different qualities of prepared softwood, the best quality is usually not too bad, try somewhere like Alsford timber or someone that stocks Hoppins timber.
Jason