7 and 8 spd cassetes have identical cog spacing, so you can also use shimano 8spd shifters.
Suntour 6 speed shifters can often be modified to index shimano 7, and are very cheap.

consider ST-2303 sti brifters, inexpensive since 8spd is nowadays considered low grade, also allows you to upgrade your wheel to 8spd cassete in the future

On second thought...
I would probably go with barcon shifters, since if this is a mtb you will most likely want dirt drops instead of traditional road drop bars, and thus will be spending most your time off the hoods.