How do I find my cost basis if I am converting / rolling over a Simple IRA to a Roth IRA when my contributions were made by my employer and myself from 2 years ago?

Your SIMPLE IRA is a type of traditional IRA.  IRAs do not have "cost" basis, but traditional IRAs can have basis in nondeductible traditional IRA contributions in the aggregate of all of your traditional IRA accounts.

If the SIMPLE IRA is your only IRA account and you never rolled any money from another type of traditional IRA or another type of employer plan where you could have made nondeductible or after-tax contributions, your basis in nondeductible traditional IRA contributions is $0 because neither your elective deferrals nor employer contributions to a SIMPLE IRA are nondeductible contributions.

You would either need to contact your former employer or use your prior year tax information to make this determination.

Basically, any amount of contributions to your Simple IRA that were considered post-tax or nondeductible would be part of your cost basis. So for any contributions that were made by you or your employer that were tax deductible on your returns in prior years would have no basis so the cost basis would be zero ($0). (This is the more common case.)