Is a product patentable that combines items available for sale in different products? How?

Without more details it is impossible to answer your question. However, gathering and packaging several pre-existing items without the prior permission of each company may not be possible. Generally craft items are not considered protectable intellectual property.

The test of invention is the originality of the discovery, and discovery depends upon the mental act of conceiving the new combination, for substantially every invention is only a combination. Nothing is more illusory, as nothing is more common, than to assume that this can be measured objectively by the magnitude of the physical readjustments required. Courts never tire, or at least in earlier times they never did, of expatiating upon the freshness of insight which observes a little, but fruitful, change which had theretofore escaped detection by those engaged in the field. When all is said, we are called upon imaginatively to project this act of discovery against a hypostatized average practitioner, acquainted with all that has been published and all that has been publicly sold. If there be an issue more troublesome, or more apt for litigation than this, we are not aware of it. - U.S. Judge Learned Hand in Harries v. Air King Prod. Co., 183 F.2d 158, 162 (2d Cir. 1950)

The short answer is no. In order to obtain a patent for your invention, it must be novel and nonobvious among other requirements. The fact scenario you have described could be used by a patent examiner to render any claims you might have for the combination obvious, and therefore unpatentable, by simply showing that one of skill in the art at the time of your invention would be aware of Craft Kit A (item X) and Craft Kit B (items Y and Z) and that it would be obvious to one of skill in the art to combine the Kits A and B to arrive at your combination, X, Y and Z. Obviousness rejections are among the most difficult to overcome to obtain a patent and there are ways in which you might argue nonobviousness. But, you would be well-advised to seek the advice of patent counsel to fully explore your facts and how you might patent your invention, assuming you can overcome what appears to be a straightforward obviousness rejection.

A product is patentable if it is (1) new, (2) useful, and (3) not obvious to persons in the field of the invention. In your fact pattern - if nobody ever combined, thought to combine, or better - thought that you could not combine the items as described - then you might have a patentable invention. If the combined invention - does what it would be expected to do, which each part simply doing the normal thing it always did, then the combination is likely NOT patentable - as it would have been "obvious" to select and combine those parts to get the expected result. Check with a patent attorney.

If the new combination were arguably unobvious to a person having ordinary skill and knowledge the field of decorating crafts and making craft kits, then yes. Otherwise, no.