The lower pulley is a ultra non-critical part. I don't mean you can do without it, but it's a simple pulley for the chain to run over and provide tension. Any pulley of the right width and bolt hole to fit will be fine. If you look around you'll find various options, and the Tacx is certainly fine.
BTW- the upper pulley is a bit different, having a calculated amount of float, so if you buy a non Shimano replacement, be sure it's a true analog with float as required.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.