Is there a general order finding quantum algorithm for a given a and N?

In general, you need to use modular exponentiation algorithm. In Qiskit tutorial, I guess they saw some pattern to for that specific case to implement operator $U$. Yet you can use the following idea to create operator $U$. Let's suppose that $a=11$ and $N=21$. u is the matrix that corresponds to operator $U$. By using u, you should be able to create a gate. Note that we are cheating since if you do all below operations, you already know the order $r$ and there is no need for order finding algorithm.

import numpy as np
u = np.zeros([32, 32], dtype = int) 

for i in range(21):
    u[11*i%21][i]=1
for i in range(21,32):
    u[i][i]=1
  

I shall explain taking the case of a = 2. The process is the same for any other value of a you have mentioned.

So to factor N = 15, you need the gates $U^4$, $U^2$ and $U^1$ gates. Where U performs the following operation $ U|yrangle = |yamodNrangle $
$ U^2|yrangle = |ya^2modNrangle $
$ U^4|yrangle = |ya^4modNrangle $

We first apply $U^4$ then $U^2$ and finally $U^1$. I am assuming that you are aware that we start Shor's algorithm by giving an input $|1rangle$ to $U^4$ which simply performs $|16mod15rangle = |1rangle$. Actually if you try out all the possible input values you will realise that $U^4$ is actually an identity operation.

For $U^2$, the operation performed is $U^2|1rangle = |4mod15rangle = |4rangle$. Now in shors algorithm the size of the register on which U acts is 4 qubits (for N = 15, as 4 qubit are needed to represent 15, size of register is $log_2(n)$). So $|1rangle$ is represented by 0001 and similarly $|4rangle$ by 0100. Therefore we need to swap 1st and third row. This is the general procedure.

Now the two possible kets which may enter $U$ are $|1rangle$ and $|4rangle$. So you need to be able to map them to $|2rangle$ and $|8rangle$ respectively. Which is a mapping from 0001 and 0100 to 0010 and 1000 respectively. Hence the first mapping demands swapping of bits 1 and 2 and the second mapping demands swapping qubits 3 and 4. This is the process of designing these gates. In your question you have talked about U gate. You can either create $U^2$ by applying U twice or by the method I have described above. Hope this helps!