The primary source is the US Census. Wikipedia has conveniently if not super consistently compiled the information in the section on
Scandinavian Americans,
Swedish Americans,
Norwegian Americans, etc.
Here's the table from the Scandinavian American wiki:
1 Minnesota 1,580,776 32.1%
2 California 1,510,541 3.6%
3 Washington 739,043 12.5%
4 Wisconsin 728,248 13.5%
5 Illinois 575,991 4.6%
6 Michigan 403,888 4.0%
7 Texas 359,360 1.4%
8 Florida 355,458 2.1%
9 Oregon 339,031 9.9%
10 Iowa 338,161 11.5%
11 Utah 333,405 14.9%
12 Colorado 291,488 5.9%
13 Arizona 281,388 4.3%
14 New York 254,474 1.3%
15 North Dakota 231,875 36.1%
16 Massachusetts 182,339 2.8%
17 Nebraska 177,522 9.9%
18 South Dakota 172,941 21.5%
19 Pennsylvania 169,294 1.3%
20 Ohio 164,005 1.4%
21 Montana 136,688 14.1%
22 Idaho 136,620 8.9%
23 Missouri 135,340 2.2%
24 Virginia 130,099 1.6%
25 Kansas 124,017 4.4%
26 New Jersey 119,267 1.3%
27 Indiana 118,989 1.8%
28 North Carolina 110,362 1.1%
29 Nevada 102,638 3.9%
30 Connecticut 100,530 2.8%
31 Georgia (U.S. state) 97,209 1.0%
32 Maryland 79,656 1.4%
33 Tennessee 75,615 1.2%
34 Oklahoma 62,145 1.7%
35 Alaska 61,259 8.9%
36 Wyoming 51,755 9.7%
37 New Hampshire 47,955 3.6%
38 Maine 44,955 3.4%
39 Alabama 43,899 0.9%
40 South Carolina 43,306 0.9%
41 New Mexico 41,073 2.0%
42 Arkansas 38,308 1.3%
43 Kentucky 34,592 0.8%
44 Hawaii 30,976 2.4%
45 Louisiana 29,175 0.6%
46 Rhode Island 26,476 2.5%
47 Mississippi 19,501 0.6%
48 Vermont 18,378 2.9%
49 West Virginia 14,519 0.8%
50 Delaware 11,232 1.2%
As you can see,
Minnesota is ground zero in terms of both relative (percent) and absolute ("total") populations. Other states ranking highly in both metrics include Wisconsin, Iowa, Utah, Washington, and Oregon. California, Illinois, Michigan, Texas, and Florida have high populations; North Dakota, South Dakota, Nebraska, and Montana have high percentages. So basically the Midwest and West plus Texas and Florida. They're otherwise poorly represented in the South and are even less represented in the Northeast.
Andersonville is a very popular neighborhood in Chicago founded by Swedes, and it retains/displays the heritage pretty well. I'm not sure if there are any prominent Scandinavian ethnic enclaves in other large American cities, but there are for sure a bunch of thoroughly Scandinavian small towns. (Investigate further
here.)
"Eastern European" could mean a ton of different nationalities/ethnicities, so you'll have to be clearer, though it's probably safe to say the answer will be the Northeast and/or Midwest, especially if you're talking about Slavic populations (e.g., Czechs, Poles, Russians, Ukrainians).