Since there are so many different styles to choose from, picking the right seat for your bike can turn into quite an adventure. There are lots of vendors who cater to Café Racer community with lots of different options. There are places in UK that sell high quality seats for a very reasonable price, assuming you live in North America the only drawback is wait time to have the seat delivered to you. Although quite few of them a so quick makes you feel like it came from another state.
There are also, lots of custom seat makers in North America to choose from. It all comes down to quality and how custom you want your seat to be. A universal seat will coat a lot less than the one tailored to a specific make and model.
This is probably the most expensive item on the list. Some seats may cost upwards to $500 yet, you should be able to pick one up for under $200. Replacing stock banana seat with a thinner racing style seat will give your bike a cool custom look. Check out your local custom bike upholstery places to see if you can get a better deal that way.
You can also replace the banana seat with a single bobber style seat and a pillion, those seats could be found online for around $50 with mounting hardware included.
Here some some good ideas for a Cafe Racer seats:
TRIUMPH SOLO SEAT SMOOTH BLACK 9.5″ w/ 4″ FLAT BLACK SPRING BOBBER CHOPPER