Re-entry Permits Laos And Cambodia - Available?

Is any kind of re-entry permits available on Lao or Khmer (tourist) visas (30day) as bough on arrival?

The thing is I expect to visit briefly several times within 30 days (single entry visa), so would like to avoid buying multiple visas as the previous visa stamp would still be valid if only a re-entry permit was available or even if re-entry is allowed automatically (free)?

To clarify what I mean:

1) Arrive in Laos 1.March 2010 & buy a 30 day single entry tourist visa. Thus immigration would stamp me in with permit to stay until 31.March 2010 OK?

2) Depart Laos 3.March 2010 & thus get an exit stamp (3mar)!

3) Enter Laos say 10.March: Now would I be able to again get a (free) entry stamp (10.mar) with permit to stay until 31.March or at least if able to secure re-entry permit before leaving Laos on initial trip (entry 1 on 1.mar)???

Seems multiple entry visas are hard to get by (even biz visa), so was just wondering if buying re-entry permit (single, double, multiple) is available?

Thanks!