There are some long out of print official miniatures, made by Citadel, Fasa, Heritage, and Wee Warriors. Last Unicorn also did a TNG-era Federation Away Team. All of those (except Wee Warriors - never even heard of that one until Lost Minis Wiki) can be found on eBay and the like, but tend to be pretty pricey! The Fasa ones at least are also pretty rough, and on the small side. Definitely 25mm, rather than 28.
Space Vixens aliens are really your only option for most non-Federation models. They are on the large side, and a bit chunky, but I like them (have some Gorn, Klingons, Jem'Hadar, and Ferengi through them).
Victory Force miniatures does some Star Trek-ish minis, to me they don't quite fit the bill for either TOS or TNG, although they also have some "alternate universe" figures which are spot-on for the movie era. In their Suburbaknights range, they also have a pack of what are supposed to be convention attendees in Klingon costumes, but I would imagine they'd work fine as actual Klingons, too.
Hope that helps!