Depends where you are. If USA, rjettek is the USA distributor and does not have anything for the complete R1900 R2000 in stock. You can only get the partial set that is meant form the R800 R1800 minus the blue color.
You will need Orange and Gloss Optimizer which they don't seem to hold in stock.
If you are in Europe there is a company that apparently stocks OCP inks. Someone here can probably provide the name and URL. Your only other option is to order directly from OCP in Germany.
So what I did was to use Image Specialists Orange and Gloss Optimizer.
The resulting Hybrid must be custom profiled in order to get a fairly good match to OEM.
Octopus-office in Germany sells OCP inks. The site can be viewed in German, English, and italian. Some info is only available in German though. Link here: http://www.octopus-office.de/en/shop/product-search/ . Enter Epson stylus Photo R1900 in the search field to see available inks, optimiser, and accessories for your printer.
Thank you very much for your answers.
I try to choose between the OCP inks and Precision Colors inks (Image Specialists inks). Any idea which of them is the best?
Both are good. IS from PC will be OEM matched to the point you will be able to use Epson's own profiles. OCP is super cheap per ml but you might need to have custom profiles made for your paper / printer combo.
Depends on your level of picky!