The Halacha states that the Sisit must belong to their wearer. This is derived from the Pasuk which says "V'Asu LaHEm"-They shall make for themselves-from themselves. Therefore, one cannot fulfil his obligation with a stolen pair of Sisit. The question is whether it is permitted to use Sisit which were purchased from the store, but not yet paid for. If the buyer continuously pushes off payment, despite the seller's repeated requests, and it seems that he has no concrete intention of paying, the Halacha deems those Sisit as stolen. If the seller is willing to wait and give the Sisit on credit, then they are valid. One should be careful when buying Sisit to pay immediately or at least clarify that the seller has no problems with the delayed payment. If the seller doesn't explicitly stipulate that he has no compunctions, it is possible that he actually does expect payment but was too embarrassed to demand it.
The Halacha also discusses whether one can fulfil the Misva if he used a borrowed pair of Sisit. The English word "Borrow" has two variations in Hebrew. It can either refer to a transaction in which the borrower actually becomes the owner of the object and is merely obligated to return a different identical item. The classic case is borrowing money. The borrower is not expected to return the actual bills or coins that he received. There is also the type of borrowing in which the actual object itself must be returned. The first type is called "Halva'ah" and the second type is called "She'elah." Regarding Sisit, it is automatically assumed by default that any time someone borrows strings for Sisit or a Tallit that the intent is to give it as temporary gift, so that the borrower becomes the owner and may fulfil the Misva.
One should insure that he pays for his Sisit in full according to the explicit terms of sale.
If one borrows Sisit, he fulfills the Misva.