I give you a lot of credit for growing Tillandsia from seed... can be quite a challenging and time consuming project (especially indoors), but very rewarding in the end. There are quite a few miniature species of Tillandsia that stay small in size. You might want to research the diaphoranthema group of Tillies; many of which range in the one inch (2.54 cm) size when mature.
Tillandsia tricholepis is a self pollinator, producing seed rather easily. Not a rare species by any means, but desirable *and in demand* by serious collectors. Some other good "miniature candidates" that might be desirable to work with: T. loliacea & T. funebris. Another possibility, T. ionantha (although not really considered miniature, but many ionantha species/cultivars are small in size), easy and fun to work with.
Here in the States, seeds are usually obtained from other growers that "are really into it". Most usually just acquire & start with plants (via offsets or etc.), grow them out to maturity, and then produce seed if wanted. I don't know what the laws (in regards to importing seeds and plants) are in the Netherlands.
The Netherlands is known (and held in high regards) for producing many high quality plants, so maybe you can reach out to see if there are any Tillandsia growers around your area that are producing seed stock (saves you the "case of Tylenol headaches" you may encounter with importing seeds/plants, especially pertaining to CITES species). One of the problems with Tillandsia seed is its "viability". If the seed is not "viable", disappointment is sure to follow. If I remember correctly, Tillandsioideae seed has a short viability of around 3-6 weeks.