Like a d100 only more so.

A d1000 is a roll used ocasionaly in some RPGs and wargames. It is almost always generated by rolling 3 differntly coloured d10, having stated each colour is either the hundreds, tens, or units digit. Alternatively, if you are a so cool as to have one, roll a physical d100 for tens and units, and a d10 as the hundreds digit. The d1000 is usualy only used for very large randomization tables, or for large tables where having significantly different ranges for different results is considered to be important.

For example Men & Supermen contains the following table for generating blood type;

001-384 O Positive

385-461 O Negative

462-784 A Positive

785-849 A Negative

850-943 B Positive

944-960 B Negative

961-992 AB Positive

993-999 AB Negative

000 Mutated Blood

Let's say i have red, green and blue d10. I say that red is hundreds, blue is tens, and green is units. I roll the dice and get 9 on the red, 5 on the blue and 0 on the units. This gives 950, which is B negative; my blood group IRL, if you care. A d100 roll would not suffice for this as AB- and mutated are so rare. Although it could be done with a d100 and subtables, it's more effort.

AFAIK a physical manifestation of the d1000 does not exist. It's possible one's been made, but it would be football sized, need a huge surface to roll it on, and probably not be fair.