Well, if you have already tried booting from the Best Buy disk and it won't, then you will need to try another OS installation disk. You can try the XP disks from your desktop, but the Vista license code for your laptop will not work with it (unless it's a Vista Business downgrade compatible one) and the license from your desktop is non-transferable. In other words, you may be able to install XP with the disks and license from your desktop, but you will not be able to activate it.
For inexpensive 2.5" drives, your best bet is probably an online retailer like Newegg.com. Brick and Mortar stores are going to be higher, but sometimes that is the only option if you are in a "need it now" situation. You could check around for a smaller computer shop. You can find some deals in those places every once in a while.
Let us know if you are able to get further in the install with the new drive.