David, garden lime is "quicklime" - hydrated calcium oxide; though I think in some cases this may be mixed with, or wholly replaced by, crushed chalk or limestone (i.e. calcium carbonate). Dolomite-rock, which is calcium magnesium carbonate, is calcined ("burnt") to form dolomite lime. So, the difference is the presence of a significant proportion of magnesium in the latter. Both will raise the pH of the soil, dolomite lime having the higher pH. The latter will also be useful where there is Mg deficiency.
Dolomite lime can be ordered over the web from GardenDirect I think, probably among other companies.