https://www.the-pillow.com.au/resources/bed_sizes is pretty comprehensive. Note that there has been a general drift to larger mattresses over time; something you bought 25 or 30 years ago can easily be non-standard now.
It sounds like what you've got are Single mattresses, which is now (mostly) subsumed into Twin as a sales category. (Two Twins is one King which is increasingly popular so each half of a couple can have their prefered mattress properties. And why make two sizes of mattress that are almost the same?)
Twin fitted sheets won't be tight but Twin flat sheets won't be large enough to let you make the bed with a bottom flat sheet; you'd need to go up a size to Double (or possibly Queen if you're trying to get over a thick mattress plus box-spring stack).
IKEA has single sheets (presuming the length matches!); I would expect a whole bunch of other places do, too, but you need to check length and width for fitted sheets.
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It sounds like what you've got are Single mattresses, which is now (mostly) subsumed into Twin as a sales category. (Two Twins is one King which is increasingly popular so each half of a couple can have their prefered mattress properties. And why make two sizes of mattress that are almost the same?)
Twin fitted sheets won't be tight but Twin flat sheets won't be large enough to let you make the bed with a bottom flat sheet; you'd need to go up a size to Double (or possibly Queen if you're trying to get over a thick mattress plus box-spring stack).
IKEA has single sheets (presuming the length matches!); I would expect a whole bunch of other places do, too, but you need to check length and width for fitted sheets.