If you have standard height retainers, then finding a +0.100" retainer could be the easiest method, but if your using Ti retainers it could also be the most expensive. One issue with raising the retainer, is retainer to rocker arm clearance.
You could cut the valve spring seat deeper, but you need to make sure the seat is fairly thick first. Cutting the spring seats is fairly easy if the heads are disasembled, but it does take some setup time, and the heads will need to be cleaned up from all the aluminum shavings.
As mentioned, adding +0.050" valve locks is not too much extra cost, and the valve seat would not need to be cut as deep.
If you can find a spring that works at the 2.00" install height, that may be the simplest and easiest route?