Gas tanks are VERY hard to find at a decent price - I mean unicorn-like hard to find. A new one is hella-expensive - something like $1500 unpainted. Used ones are, naturally, expensive also since the sellers know what a new one costs. Even badly dented up used ones can go fo over $1000. Best of luck finding one.
For handlebars, there are often takeoffs available used where someone has replaced the stock ones with some aftermarket bars. You can also buy aftermarket bars but most of these will have the wrong knurling spacing since the vast majority of aftermarket bars are designed for Harleys which have different spacing from a VTX. You can install bars with the wrong knurling but this may require a little extra work and/or caution.
It also matters what model VTX1300 you have. the bars for the R, S, and T models are all separate from the stock risers. The C model has the risers welded to the bars making them harder to replace.