First of all, welcome to the forum!
You can always buy a new hydrometer. Once gravity is constant for 3 days, fermentation is complete and your beer can be bottled.
However, if you want to make the best beer possible (and who doesn't) you should let the beer sit in primary for at least 2 weeks, 3 is better. This gives the yeast time to clean up after itself, helping to get rid of any byproducts from fermentation that can cause off-flavors. After 2 -3 weeks in primary, fermentation is almost certainly complete, and you can bottle safely. Then let the beer sit in the bottles for 3 weeks, chill 1 or 2 for 24 - 48 hours, and try it. Buying a new hydrometer should still be a priority. It's really the only way to know for sure when fermentation is complete.
Time and patience are your friends in this hobby. The last beer from the batch will be the best beer of the batch. In the meantime, get another batch going so you have something to occupy yourself with while you wait!