HCA: Hermit Crab Association

A few shell questions:
We purchased a pre set up terrarium with 3 PP. There were no extra shells, so we need to buy some.
1. If the crabs burrow most of the time and can't measure them do I just guess the size? Or how do I know?
2. Should I buy shells that are similar shapes to what they currently have?
3. Are the shells from petstores any good, or is it better to buy online somewhere?
4. Where online should I look to buy shells? They need to deliver to Canada (for a reasonable price)
Here are pictures of my 3 crabs in their current shells.
https://photos.app.goo.gl/NRRuLLjFYBz2pFxB8
https://photos.app.goo.gl/gN8VoG8vsFyz53Mu6

1. You can guess sizes of shells but expect to have a lot of wrong sizes shells. If you read a few topics about shells it happens a lot, sometimes two or 3 times. You are better off waiting until you can measure then pick shell approximately 1/8" to 1/4" larger.

2. You can get different types of shells. Most crabs prefer Turbos although some will pick different types. There is no telling what they will pick or even switch at all.

3. A lot of pet shops sell shells for crabs that they can't even fit in. Some only sell painted and others have a shiny coating on them. You can sometimes luck out in craft stores or at Wal Mart. You can usually find bulk bags of shells. You can usually see a few good type shells, if they are the right size.

4. There are several links to online stores in the "Navigation" section on here. Look at the "Shopping Resources". I believe some places are noted as "ships to Canada"

Hope that helps.