Is it possible for a PC to create a treant without using a spell like wish or miracle or an epic spell?
Spells that create a treant…
The 7th-level druid spell changestaff [trans] (Player's Handbook 208) allows the caster to turn a specially prepared walking stick into a virtual treant. Similarly but less portable, the 6th-level druid spell liveoak [trans] (PH 248) creates an ersatz treant guardian when a druid's near a sacred space.
More frighteningly, the 9th-level wu jen wood spell arboreal transformation [trans] (Complete Mage 95) for 1 day/level changes a creature into a treant that obeys the wu jen's commands; when the duration expires, the creature becomes a normal tree.
…And, as alternatives, magic items that produce a treant
The sapling club (Arms and Equipment Guide 115) (32,450 gp; 3 lbs.) is a +1 club that the wielder can command to become for up to 12 rounds/day a treant, and the seeds of the treant (AE 136) (15,300 gp; 0 lbs.) brings forth from the planted seeds instantaneously a for-reals treant… over whom the planter has no control. (Apparently the Guide had a serious thing for treant-themed items. I mean, also see the staff of trees (AE 127) (120,000 gp; 4 lbs.). Who knew?)
Liveoak (Druid 6):
Liveoak must be cast on a healthy, Huge oak. A triggering phrase of up to one word per caster level is placed on the targeted oak. The liveoak spell triggers the tree into animating as a treant.
Changestaff (Druid 7):
You change a specially prepared quarterstaff into a Huge treantlike creature, about 24 feet tall. When you plant the end of the staff in the ground and speak a special command to conclude the casting of the spell, your staff turns into a creature that looks and fights just like a treant.