Do you get new uniforms when you join the reserves after leaving acting duty?

Yes you can. If your uniforms are unserviceable then the supply sergeant can order you an initial issue of 2 uniforms. They can also order you the name tapes and everything. But it is all subject to availability of funds. They can place the order but if there are no funds it will just wait in the queue until funds are available.

If your current uniforms are serviceable, then no. But you can turn in unserviceable ones and get new issue. However, this is interesting timing for you. Soon reserve units should get funding to start ordering the OCP pattern. If you have not already made the purchase from your clothing allowance, then you should get the new stuff with the rest of the reserve unit.

Your supply sergeant should be able to give you specifics. If you have not already, when you in-process to the new unit, you will be asked (or should be asked) to fill out some sort of sheet with all your uniform sizes so that your supply chain can put you in the KYLOC (KY Logistics Operational Center) system for managing your clothing bag order.

SPC(P) Worthington, I am unit supply in a Reserve unit. You will not be issued new uniforms as you are required to keep all uniforms upon release from active duty. I refer you to AR 700-84, para 12–2 (b).
Personnel with more than 3 years active duty
a. Active Army personnel with more than 3 years active duty who are discharged under honorable conditions may retain all clothing items in their possession. Voluntary turn-in of all recoverable items is authorized per paragraph 12–10.
b. Personnel who are REFRAD and transferred to ARNGUS or USAR units will in every instance, retain all personal clothing in their possession. Voluntary turn-in of these items is not authorized. DA Form 4886 will be used to establish accountability (see para 12–9c and fig 12–1, and the Reserve Components personnel update).
c. Personnel who are REFRAD and transferred to USAR control groups will retain personal clothing per table 12–1.

At the Reserve unit level, we know you receive a clothing allowance and that upon discharge you are given the prorated clothing allowance depending your date of discharge. I would highly encourage you to use that money to get your OCPs.

In the Reserves, our uniform funding is shaky at best. I've had orders in for the new PTs since April of 2016 and haven't seen them shipped. I have orders in for OCPs for three months and the funding hasn't been approved.

You might get lucky, but from my experience I suggest you take care of your uniforms before you discharge.