Documentation requirements can vary a little bit by local visa office. If you are submitting to Colombo, I see nothing in the visa office-specific instructions about submitting pay slips. The contracts and letters of reference are required, so do your best to get those.
Some visa offices request payslips, but not all. Some people submit payslips and/or tax returns as substitutes for the required documents (e.g. reference letter fails to mention salary, and all attempts to get a revised version fail). Sometimes it helps, sometimes does not, depends on the rest of your documentation. As well, self-employed persons have to provide extra documentation. Don't get confused, this does not apply to everyone. Follow your visa office-specific instructions.
For your Store Manager position in Sri Lanka, I would submit a certified copy of the old letter, along with your explanation of why you cannot get an up-to-date reference. They may or may not give you credit (points) for the experience, but you will have met the documentary requirements to the best of your ability. Hopefully, you have enough points without this one year.
As for the others... if they are from the US, it is the job of the HR people to provide references confirming employment details, but the actual contracts and payslips... I doubt that. You don't need the payslips anyway. As for the contracts, you would have to hope that your immediate supervisor might still have copies, and be sure that your reference letters include all the details that CIC asks for.
If you have absolutely no other evidence of salary, I would ask your former US employers for copies of your annual 1099 statements (the employer-issued statement of income paid to the IRS, that you have to file with your tax return). You CAN request these from the IRS, but it takes time, and they prefer that you ask your former employer first. They should have these on file.
