Your dilemma is shared by many other stitchers. There is np floss sold that will be an exact match to the floss that is used by Dimensions. I would just go and buy floss that is colorfast,. easily available (DMC or Anchor) or the recently more available. I just replace all of the kit floss with one of these bramds.
The other way to deal with this issue is to call this manufacturer...be ready with their stock # for the kit amd the Dimensions name for the colors you are short. Be patient.
This problem occurs often with some kits.

This is a Dimensions kit and they use their own floss. If you want to know what is close to their floss colors you can go to Cyberstitchers.com. They have charts for different flosses for comparison. I know they have one to compare their Dimensions floss to DMC floss. Though it won't be exact it will be close. Hope this helps.

Thanks for cross reference site. Just what I needed!

Out of light grey #18510 for kit #8740

appletosh - this is a Dimensions kit and they use their own brand of floss for their products. The only way to get replacements for it is through them directly if you don't want to find a substitute on your own. In a post above yours, a fellow stitcher gives a website you can check out to see what alternatives are out there. Or you can just contact Dimensions and get a replacement. Here is the link to their Contact Us page on their website: https://simplicity.com/contact-us/

Hope this helps! Thank you.
~Kristi @ ECS