As others have said, Chiltern Seeds is a good source.

Perennials that are easy from seed - Lychnis coronariia in pink or white, and the bright red "Maltese Cross" thing. Ragged Robin - Lychnis flos-cuculi, makes a very nice garden plant if your soil is on the moist side. Malva moschata, both the pink form and the white form - seeds itself around very happily, and has lovely finely divided foliage. Foxglove - biennial, so plant a few seeds two years running. Other Digitalis species also grow easily from seed.

Purple loosestrife - tall spires, available in shades of red purple. Mullein - another biennial, with felty leaves and yellow flowers.

Most wallflowers can be grown as perennials, especially if you trim them back so as not to get too straggly and provide welcome scent early in the year.

The british field geranium has big blue flowers and self seeds readily - there are a lot of garden varieties, but the wild type is garden worthy. Some of the other hardy geraniums can also be grown fairly easily from seed.

Don't forget cuttings as a means of propagation. In it's most basic form (ie the lazy way, not necessarily the most successful way) - cut off a twig in mid-late summer, just beneath where a leaf comes out. Remove all the leaves except 1-2 at the top. Stick it in a pot of soil so that it is at least 2/3 submerged. Take a dozen or so cuttings - as many as the pot will hold. Make sure the soil is good and moist, then enclose the whole thin in a plastic bag. Every month or so remove any cuttings that have rotted. By the following spring there should be roots coming out of the bottom of the pot. Fuchsias (there are hardy varieties) and pelargoniums are easy ones to start with.