I get mine from 3Rivers. The bag is labeled Insert, Aluminum, - Heritage 75/90 InsertD-10 (A-16-C-11, not sure what this set of numbers/letters is, except they are 11 gr). They do seem a little looser than many inserts I've installed in shafts - some a little too tight for comfort (when I worked in a pro shop). I use super glue and have never had a problem with them pulling out in targets (or other less desirable places - like the occasional stumps, trees, etc - LOL)
I found them on Pg 59 of their current Master Catalog, at the top of the middle column, with a listing of other inserts. I'm pretty sure i also found them on their website the same way. I wouldn't swear to this but it seems to me I called them once about something and asked about these inserts for my heritage 90 shafts also, and got a positive answer. I've ordered them from 3 rivers at least twice and perhaps three times.
Hope this helps. If you have doubts, I would call 3Rivers (a sponsor here) and talk to them - they are always ready, willing, and able - as well as friendly, in my experience.
Oh, man, I wrote all this (because i had the same problem at first), and then realized you were asking about nocks. I'm sorry. However they also are listed in that same catalog - on page58, in the center column, under TCX Heritage Nocks. I've also ordered/used those a couple of times. (Again, sorry but sometimes I'm a real mess - and deer season does open tomorrow, after all - :D )