I get that, but here's how you can overcome it:
1) Figure out what you would pay per month if you financed it.
2) Open up a savings account. It'll ONLY be used for this purpose - once money goes in here, it doesn't come out until your savings project is finished. (With the obvious caveat that, if you have an emergency, you can tap this fund.)
3) Every month, put whatever your payment would have been into this account.
Now, instead of paying interest, you're actually earning interest. And when you're done and pull the trigger on the lights, you won't have to remember to pay anything - they'll just be paid for.