The hefsek on Shabbat remains valid even if you weren’t able to perform a bedikah on Sunday. However, Sunday may not count as day 1 of shivah neki’im if you didn’t perform any bedikot that day. If you were able to perform a bedikah on Monday that counts as your new day 1 and you are scheduled to immerse Sunday night.
For the future if you don’t have any bedikah cloths, you can make your own from any clean, soft white fabric. Cotton knit is often the most comfortable. For example, you can cut up an old clean white undershirt, or an old pair of white underwear, or a soft, well-washed handkerchief. You can even perform a bedikah by inserting a portion of an intact garment, such as a pair of underwear.
Please feel free to get back to us with any further questions.
B’Hatzlacha!