You need to look at your frame geometry and determine what travel suspension fork it is designed for. Older XC frames had 80 to 100mm travel. The latest generation XC/Trail frame have 100mm travel to 130mm. (I believe that 120mm has become the most common for modern rigid hardtails.)
With the above being written, the most common 29er rigid fork I have run across fit 100mm wide hubs, have a 45mm offset, and come in 470 to 490mm blade lengths. These generation forks can be purchased for around $100.
Then there are the new generation 110 boost forks with 45mm offset that most often have 480mm fork lengths and these days cost $200+ each.
(I found that for 120mm travel frames, carbon forks with 480mm blades match the frame geometry best.)
There are so many forks listed on Aliexpress, I recommend searching under carbon mtb forks and start studying each forks geometry.