Neither. Toi is a stressed pronoun, so you only use it in certain specific ways, such as after a preposition.

In your sentence, assuming you mean “I hope this card finds you well,” you need the direct object pronoun te and it has to go in front of the verb:

J’espère que cette carte te trouve bien.

the word order should be :

J'espère que cette carte te trouvera bien.

(where “toi” is replaced by “te” and placed between the subject (cette carte) and the verb (trouve) )

However, this type of sentence sentence doesn’t sound good in French, I would rather write :

J'espère que tu recevras bien cette carte.

or

J'espère que cette carte te sera bien transmise/remise. 

if you have some doubt that the message could get lost.

EDIT: if you want to say that you hope that the person reading the letter is in good health, you can simply say :

J'espère que tu vas bien (or "que vous allez bien" if this is a formal letter).