"could sought" in a conditional statement

Question

I am asking my friend for a favor. If he doesnot accept then i have to inform him that i will look for someone else to favor me. For this context, i am confused with using could and should or would in future sense.

If not, reply me so that i could sought for others!

Did the sentence conveyed what i expected?

I am little bit confused about future perfect tense and present perfect tense!

Thanks in advance

---

Update based on answers

So

Could is the 'past' form of can. It expresses past tense in 'real' mode, or non-past tense in 'unreal' mode, but it cannot express non-past in real mode. You have to use can.

But the next answer suggests

If not, reply to me so that I could seek help from others

I will look for next person if he denied and i am not sure whether he will deny or not. So that could seek depends on a event that has yet to occur. So following infinitive form of the verb after could expresses the context in present or future ?

Answer 1

1. There are no perfects here - the perfect is constructed with the past participle preceded by a form of have.

2. A modal (can/could, may/must, shall/should, will/would) must be followed by a verb in the infinitive (plain) form: seek in this context. But seek is very stiff and formal - in ordinary speech we say look for

3. Could is the 'past' form of can. It expresses past tense in 'real' mode, or non-past tense in 'unreal' mode, but it cannot express non-past in real mode. You have to use can.

4. Reply requires the preposition to for the person addressed. Again, it is pretty formal; in ordinary speech we would say tell me or let me know.

5. Others, too, is pretty formal; we would say someone else or somebody else.

So what you want to say is something more like:

If not, let me know, so I can look for somebody else.

Answer 2

It would be better to say:

If not, reply to me so that I could seek help from others!

You can replace help with a favor if you like.
There are several changes here, but regarding your main question, you cannot use sought because the action help is not in the past, as it has not happened yet.

Also, since this is a reply to your request, an exclamation point (!) may not be appropriate.

Answer 3

After a modal verb such as can/could only a bare infinitive or a bare infinitive perfect can follow. Other structures are not possible.

Possible: can seek, could seek - can have sought, could have sought

Not possible: can sought, could sought

By the way, you could say: If you don't give me an answer I'll ask other people.

Answer 4

Or, more elegantly: "if not, please reply, so that I might seek [the favor] (of/from) someone else." (This is a bit formal, but actually more apt than "can" or "could" since you could seek the favor of someone else whether or not the first person replies.)