Here are some resources that you might find useful. Laypeople should be aware that it's possible to make all sorts of completely illegal and ugly achievements, but such will not be approved by the Coletx.
-NRH
(https://cdn.heraldicon.org/img/heraldicon-logo.png) (https://heraldicon.org/)
Heraldicon (https://heraldicon.org/) (free) - Currently, this site is the best option. It has a ton of art in different styles, and you can also upload any SVG (like from the free Noun Project (http://thenounproject.com)) to add more. It is extremely customizable and allows fine adjustments, and it can create achievements from a provided blazon (or vice-versa). The only downside is that there's a bit of a learning curve.
(https://drawshield.net/img/jumbotron.png) (https://drawshield.net/)
DrawShield.net (https://drawshield.net/) (free) - This site has an excellent selection of art and uses proper blazons, but sometimes it requires a lot of fiddling with the specialized language to make something look right. It's an old standby, though, and great for a quick mockup.
(https://myblazon.com/app/src/assets/images/lion-ecu.webp?v=IKzcEURUcIEHnHX6ISHUe2mXQhkDcAZAqW1Hh2oUMvA) (https://myblazon.com/)
MyBlazon (https://myblazon.com/) (free) - Another site, but with poor design tools, a limited art range, and clunky outcomes. Only useful if you want a smooth integration with product ordering, but probably not even for that.
(https://cdn.cloudflare.steamstatic.com/steam/apps/1152420/header.jpg?t=1676741731) (https://store.steampowered.com/app/1152420/Heraldry_Studio/)
Heraldry Studio (https://store.steampowered.com/app/1152420/Heraldry_Studio/) ($19.99) - This program works decently well and has a good amount of included art in traditional styles, but it's also surprisingly limited considering how long it's been in development and the overall price.