This is a hard question to ask, as well as hard to answer.  When is it a good time?  Is it ever a good time to let go of that loved one?  When will you be ready to let go of a pet you see as someone who is a part of your family?  With this being such a hard question to answer I guess it is more important to cover how to prepare, if you believe in in having your pet put to sleep. 
Everyone one deals with this in different ways and it is best to read of all the different alternatives depending on you.  Our suggestion is do what is best for you.  No one person is the same as the next and no one knows you and your needs better than you.  Many times people deal with letting go of an older dog buy getting a new puppy.  Many other people need to have just a little grieving time as well as other need years to recover from such a loss.  Finding the perfect new pet can sometimes appease the feeling of loss, and this is also a popular way of coping with the emptiness that is often experienced. 
Many people start a group or attend a grievance group which is a great way to connect with others who are experiencing the same feelings and loss that you may be feeling.  Many also attend these groups before the loss of the pet to prepare for the inevitable end.  No matter which option you feel is best for you getting started looking for a new puppy for sale when you are ready is best to taken at the pace that is most comfortable for you.  Rushing into the process of getting a new puppy or a new dog could be hard to deal with if you are not emotionally ready to take those steps.  Making a decision to replace a loved pet is hard and should never be done hastily.  Finding a new pet can be easy and simply but it may be hard to deal with a new puppy with a new attitude and disposition then the pet you were once used to having at your side. 
Having a great breeder to deal with can make this process easier.  Speaking with them letting them know what it is that you are in the hopes of finding can help them recommend the best puppy for you to have.  For anyone that is looking for a breeder we suggest that you speak with several before making your decision and as with anything most breeders will be more than happy to answer questions before you make your final decision.