I use the National Park Service web site a lot. At the top level, it can help you find a national park near home or in whichever state you have interest. Then that can take you to the web site of that specific national park. Most of the national park web sites have a View Map button right there.
Now, in some cases you can view the park map on the screen, and if you try to print it out, some funny things happen with aspect ratio. In other cases you can view the park map and print out the PDF right there. Most of these are the general road maps of the park.
If you require a detailed topo map, you can get those from the USGS web site. However, few of these are of an entire park at once, at least for the larger parks. In most cases that I've seen, you can download a half-dozen quadrangle maps that should cover your area of operation. In some cases, these quad maps are of the twenty-year-old format, and in other cases, these quad maps are of the new format which appear more like satellite photos with topo features added in.
That is still likely to require you to overlay your own trail information on the quad map. If that is insufficient, then you need to purchase topo maps of the park, and there are several companies who publish those. Figure on $10-12 a pop.