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If its just the x-hair ones then usually 12mm and 9mm. For your platform Mike 12 would be enough. Besides if its not then you can always barlow it.
Mine was about $150 from Bintel but that was 12 months ago.
As for aligning your platform: This is the technique I use to rough polar align my Losmandys
1. Just ball park the north and south using previously determined landmarks. If none known then magnetic north/south will do. My mag dev is around 11 degree so I "nudge" it a bit further east.
2. roughly set my alt (though for me that doesn't change much once initially set up)
3. Slew my scope to 0 deg dec and close to the meridian. and center a star in the reticle.
4. Watch how quickly the star drifts in dec. (Ignore RA drift) If you are a long way out the star will drift pretty quickly. If it does then crank the Az adjust (or move in Az) at least a degree or two. Slew to recenter a star again and watch how quickly and in which direction it drifts.
If it drifts slowly only over a period of time and in the same direction as previously then you are nearly there. Repeat with a smaller Az adjustment in the same direction.
If the drift is in the same direction and quicker then you need to adjust your az in the opposite direction the same as your initial adjustment and then some more.
If the drift is in the opposite direction but only slow then you have to go back in small increments.
The idea is to stop the star drifting off the Dec cross hairs as much as possible. BTW initially try to line your reticle up so that the cross hairs are in alignment with the ra and dec directions.
Once you've minimized the drift at Az, move to Alt adjust.
5. Center a star about 15 - 20 degrees above either the Eastern or Western horizon close to 0 dec. Watch for drift. When the star drifts adjust your alt adjust to bring the star back to the center of the line. You can ignore any RA drift. Keep drifting and adjusting until there is no more dec drift.
6. Slew back to the meridian at 0 dec again and repeat the Az adjustments again. You shouldn't need much adjustment but if you've moved your Alt more than a couple of degrees then you will most probably need to adjust your Az.
I don't know how accurate your platform is Mike or how accurate your RA drive is but using this method I was able to take 3 min unguided images of Andromeda through my Orion on Saturday night. I did add a few more iterations of AZ and Alt though just to fine tune it. Hopefully piggybacking and manually guiding your should be able to achieve something similar. If you can get at least 1-2 minutes of the star not drifting off your reticle center line you would be able to expect at least that if not longer with no rotation.
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