Seal and few other companies market tissues made for RC. I use one but I only used it with FB paper. The instruction that comes with it specifies what temperature range must be used for each type of paper. I *think* the only difference is, RC types melt at lower temp than FB only counter part.
I have never seen pre-cut sheets smaller than 8x10. B&H does have on listed for 5x7 but it is discontinued. I'd just buy 8x10 and cut it in half. Probably cheaper that way per sheet as well.