Creating any magic item has a gold cost that includes buying requisite materials. The exact nature of the materials is left up to the group/DM, but they typically include incense, scrolls, and most importantly, the item to be magically-enhanced. Unless the mundane item has important value as a mundane item (e.g. weapons, armor, possibly special tools, perhaps intrinsically valuable as artwork in its own right), there is no separate requirement to buy an item to enhance: the cost of that item is included in the cost of the magic-item creation.
Making mundane glasses would typically take Craft (glassworking). The Craft skills tend to have a bit of overlap: Craft (glassworking) doesn’t just cover making things with glass, but also incidental skills important to working with glass. In this case, I would certainly rule that this would cover the frame or harness for the lens, being relatively simple wire-bending and/or leather stitching in comparison to the fashioning of any optical lens.
But you would only need to do that separately if you wanted your glass that can cast detect magic to also have its own mundane properties, for example if the lens also worked as a magnifying glass or something.
Finally, I’d strongly consider asking your DM to port in the artificer’s monocle from D&D 3.5’s Magic Item Compendium for this character. Its physical description exactly matches what you’re looking for, and it’s strongly related. It doesn’t cast detect magic itself, but what it does do is allow you to use identify for free if you study an item for a minute with detect magic up. Very, very useful item. D&D prices it at 1,500 gp; the magic-item-pricing guidelines suggest that a detect magic at-will item at 900 gp (less if you have some daily limitation on it), and combining the two has a 50% surcharge on the less-expensive item so that’s a total of 1,500 gp + 900 gp × 1.5 = 3,150 gp. A tad overpriced, in my book, but not bad and it’s a really useful item, able to detect magic and identify basically at will.