That Okaya RC circuit at Farnell, might be an impregnated paper capacitor since there is no mention of the dielectric in the datasheet. While undoubtedly better than those from RIFA, the only reasonably reliable way to use paper in a capacitor is in a hermetically sealed glass or tin can using choice materials and a bit of luck. While some plastic foil capacitors are cheap and unreliable, on average/on a technology level they are much better than any but the most expensive paper capacitors. Those Russian paper ones linked by Argus25 are probably quite good.
After reading the manufacturers documentation
https://www.okayaelec.co.jp/dcms_med...eg1704_all.pdf from page 52 onward, there's a lot of useful information also on selecting the right value but not a word about the dielectric. I'd say a 50/50 chance on polypropylene or paper.