You buy them from your bank where you have your bank account. On the bank website there's sometimes an "order checks" form, and you can definitely get them if you go to the bank in person - though you do have to wait about 2 weeks before the checkbooks show up in the mail. The cost is usually 20$ for a few hundred checks.

If you really only anticipate needing one or two, you can have your bank print off what's called a "money order," which is like a one-time check, but those have a fee associated (usually 5$) and you have to go to the bank every time you want one.