Reddit mentions of Rollerblade Maxxum 90 Inline Skates - 10.0/Anthracite-Green

> I looked a bit into skate sizing, but I wear large shoes (12.5/13 depending on brand). Should I shoot for the same size or higher? Are skates that size rare/expensive?

I also wear size 13s (and can get into some brands' 12.5s).
Skates at that size aren't as common as, say, size 9s, and a number of skate models unfortunately top-out at size 12.
That being said, there are a fair number of models across brands that accommodate size 13.

The thing about sizing skates, is to measure the length of both feet, and hav the actual measurements in millimeters, then compare to each manufacturer's sizing charts.
See here and here for collections of manufacturers' sizing charts.

> I can't seem to find any dedicated vendors around my area. Are skates on Amazon widely legitimate? Is Dick's overpriced, or have a good selection? What's the best online store?

The skates themselves are equally "legitimate" (or "illegitimate"), without regard to the vendor from which they are purchased - e.g. there would be no functional differences among, say, Maxxum 90s from Amazon, Maxxum 90s from Dick's, and Maxxum 90s from Inline Warehouse.
It's less about the vendor who sells the skate, and more about the brand that produced the skate.

That being said, specialty skate shops like Inline Warehouse, InlineSkates.Com, ProSkaters Place, Shop Task, and a number of others will generally have a greater variety of skate models from which to choose, will also tend to carry any spare parts or maintenance materials you may need, will generally have comparable or lower pricing, and may be more flexible (to a point) on their return policies.